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Note: Much of the information below also applies to ski goggles.
Why Wear Sunglasses:
Sunglasses are important especially in the spring and summer on the water or snow to protect you from snowblindness, and other types of eye damage.

• Comfortable vision. The sun's brightness and glare interferes with comfortable vision and the ability to see clearly by causing people to squint and the eyes to water. Light over about 3,500 lumens is uncomfortable.
• Improve sports performance.
• Prevent photokeratitis (snow blindness), which is a temporary but painful sunburn of the eye's surface.
• Reduce the risk of other eye problems, cataracts, macular degeneration and pterygiums.
• Quicker dark adaptation when transitioning to nighttime or indoor light levels.
• Protection of skin around the eyes from skin cancer.

• UV blocking: Ultraviolet rays can increase your risk of developing problems such as cataracts. Color doesn't affect UV blocking. They should block 95% or more of UVA and UVB; Opthamologists recommend 99.75% blocking. Up to 80% of UV radiation makes it thrugh a cloud cover, so UV protection is important even on cloudy days.
• Light Transmision Factor - LTF or VLT: General purpose sunglasses transmit from 8 to 40% of light [LTF/VLT]. Sunglasses range from 9-70%; Most people choose sunglasses in the 15 to 25% range. See Smith table below.
• Polarization to reduce reflected glare.
• Wire frames are lighter, nylon are more flexible.

• Metal hinges are more durable than plastic.
• Side protection - Wide plastic temples and wraparound lenses give more protection from dust, snow, wind, rain and glare that may come in from the side.
• Lens material - Glass, Polycarbonate, CR-39, High-index, Trivex. See eyeglasses.
• Lens quality
• Lens coatings - Scratch-resistant, Anti-reflective, UV protection. See eyeglasses.
• Fit and comfort
• Some sports models have "air dams" molded into the frame that direct the flow of air over the inside of the lenses or small ventilator holes to reduce fogging.
• Cosmetic - Different lens shapes conpliment different face shapes.

Lens Tints: Click on sport for more information below.

Lens color														Comments
Gray		x	x		x	x	x							all-purpose
green				x			x	x					x	varying light conditions
orange	x								x	x	x	o		cloudy, hazy or foggy conditions
Gold & yellow	x		x		x		x	x	x		x	x		overcast flat light
Amber	x								x	x	x	o	x	contrast, depth perception
Brown or Copper	x	x	x	x	x		x				o	x	x	Enhanced depth perception, hazy sun
Melanin (uvex)			x	x			x				x			High contrast, UV protection
Ignitor (Smith)	x						x				o			highlights shadows / increases color definition
Violet	x						x				o			contrast
Vermillion	x								x			x		Depth perception
Rose	x										x	x		Highest contrast - low-light
Red	x		x						x		o			Contrast, early morning or evening
Lens color														Comments
Mirrored	x													high altitude
Rose Platinum -Smith goggles	x												x	Reflective mirror coating to reduce glare in bright sunlight, while the tint continues to increase definition and still provides low-light contrast.
Vermillion Gun -Bolle goggles
Pewter/Ice Iridium			x											Okley uses iridium for their mirror effect.
Light Blue					x		x	x						Increase in contrast for better depth perception in low light or hazy conditions.

x - Recommended; o - Optional

Color: (Numbers in [ ] are transmission level.

Gray	Offer the least amount of color distortion; good for all-purpose use and clear days. allows true color perception; popular neutral color; good general purpose tint; does not enhance contrast; good tint for golf, cycling or running. Gray 3 [20%] reduces the maximum amount of visible light. Good for bright sunny days and heavy glare situations. Best uses include driving, deep-water fishing and general use. Gray 1 [41%] a lighter shade - Good for partly sunny to bright days.
green	Wavelength Enhanced: 505nm - 575nm Has slightly better contrast than the gray colors, but is not considered a high contrast lens. Green maintains true color balance and is a good choice for varying light conditions. allows true color perception; popular neutral color; good general purpose tint; fair contrast in low light; reduces eye strain in bright light.
orange	Wavelength Enhanced: 625-675nm Block blue light, offering a brighter view on cloudy, hazy, or foggy days. Good for hunting, biking and skiing. blocks blue light; brightens cloudy, hazy or foggy days; world looks yellow or orange; excellent for contrast; minimizes eye strain; good tint for sportsmen, pilots, skiers.
Gold & yellow	Wavelength Enhanced: 585-625nm Add contrast; best in flat and dim-light situations. Yellow provides the maximum light transmission. Popular for hunters and for night driving. excellent depth perception and contrast in low light; ski industry standard for overcast days;
Amber	Wavelength Enhanced: 650-700nm Block blue light, offering a brighter view on cloudy, hazy, or foggy days. Hunting, Golf Tennis.
Brown, Copper, Bronze	Wavelength Enhanced: Bronze: 650-700nm; Brown: 650-685nm Best for enhancing depth perception. better in hazy sun; enhances contrast; good tint for high-glare sports, such as skiing, fishing or sailing. Brown 3 [15%] - excellent contrast and improves visual acuity. Good for bright sunny and varying conditions. Reduces blue light. Best for driving, golfing and shallow water fishing. Brown 1 - is a lighter shade. Improves contrast. Good for partly sunny to bright sunny days.
Melanin	Blocks high amounts of blue light, while maintaining true color balance. Melanin provides high contrast for better visual acuity and is good for bright sunny and varying conditions. Great for golfing, driving and fishing, or any one with macular degeneration. Melanin is a pigment in your skin, eyes and hair that provides natural visable light and UV protection. It absorbs the higher energy light more strongly than the lower energy light. So it absorbs UV more than blue, and the blue more than green - and so on. It provides near optimum protection to the retina by filtering the different colors in proportion to their ability to damage the tissue of the retina. See: MelaOptix.com
Vermillion	Wavelength Enhanced: 675-700nm Excellent depth perception in low light; contrasts objects against blue and green backgrounds; good tint for skiing or sportsmen..
Rose	Wavelength Enhanced: 625nm - 700nm Has the highest contrast and best low-light image resolution.
Red	Increases contrast. Often used for fishing in early morning or late evening hours. Also used for skiing and hunting.
Violet	Wavelength Enhanced: 400nm - 450nm increases contrast and dampens certain backgrounds. Violet is often used by hunters in average or bright conditions. Also used for skiing, snowmobiling and golfing.
Mirrored	Reduces the amount of light that reaches the eyes; good at high altitudes.
Gradient	Shaded from top to bottom. (A double-gradient lens is dark at the top and bottom, and lighter in the middle.) Driving glasses are often gradated so that you can see the dashboard clearly.

UVA (320-400 nm)
UVB (290-320 nm)
UVC (200-290 nm)

Activities:

Golf	Copper-colored	Improve depth perception and make the contours of the ground more distinct. Golf frames - some manufacturers offer special frames with minimized bridge and extended bottom flaps to reduce the visual distraction of the frames during your swing
	Green	slightly better contrast. Green maintains true color balance and is a good choice for varying light conditions.
	Violet	Helpful finding the ball in the rough.
Tennis	blue or green	Allow the blue wavelengths to pass through the lens, highlighting the background and enhancing the contrast with the yellow ball.
Snow skier or snow boarder	Browns, coppers, ambers	Excellent low to mid light lens, amber lenses filter out blue light to improve contrast, and increase depth perception in varying light conditions for better definition of contours and textures of the snow's surface. Plastic frames - sturdier and more flexible than metal frames Wraparound frames - offer the best protection against wind and snow. Polarized lenses - absorb glare off the snow, although some say you will not be able to see ice with them and they reduce contrast. There doesn't seem to be any general agreement on this.
	Yellow, Oranges, rose and red	Help the skier see shapes, objects and bumps in the snow more clearly because they help block out blue, or hazy, colours that are common. Often, these lens tints are called "blueblockers" because of this blocking ability. They work well on overcast days
	Mirror	effective because they limit glare.
Boaters and fishermen	neutral gray	Enhance dark fish and are best for deep water fishing. Amber lenses accentuate sandy bottoms of shallow water
	Brown	bring out grassy water bottoms
	Yellow	is the most effective in low light or highly overcast situations.
	Red	increases contrast. Often used for fishing in early morning or late evening hours.
	Blue	Best suited for open ocean.
Hunting	yellow or amber	Light up the viewing area along with providing contrast.
Cyclist	mirror	Reduce reflections Wraparound frames - offer the best protection against wind and dust
	High-contrast browns and coppers	Enhance the perception of the road or track surfaces in bright light
	Yellow	Most effective in limited light conditions and is superior in contrast enhancement.

Sources: Sunglasses for Sports - Choosing the Optimal Lens Color at EyeTopics.com
Choosing Ski Goggles at SimplyPiste.com

Glacier glasses (special sunglasses designed specifically to protect your eyes from the intense light at high altitudes) have a VLT/LTF of around 4-10%. Most glacier glasses also have shields to protect your eyes from light coming in from the sides of your lenses. Glacier glasses should not be used for driving or other everyday activities.

• Use a soft, line-free cloth (such as a microfiber) to clean your lenses.
• Avoid wood-based materials, such as facial tissue, which are too abrasive for safe cleaning.
• A spray-on lens cleaner or just plain water should also be used, since dry lenses are more susceptible to scratching when rubbed.

The Australian radiation protection and Nuclear Safety Agency (ARPANSA) has an EPF scale from 1 - 10 with 9 and 10 blocking most UVR.

HEV - High energy visible light - Recent research indicates that Blue (HEV) light might contribute to macular degeneration, or loss of vision detail.

Pterygium - An abnormal mass of tissue arising from the conjunctiva of the inner corner of the eye that obstructs vision by growing over the cornea; it arises from irritation of the pinguecula (a slightly elevated elastic tissue deposit in the conjunctiva (a transparent lubricating mucous membrane that covers the eyeball) that may extend to the cornea but does not cover it.)

Lumens:
The brightness or intensity of light is measured in lumens. For example, when you are indoors, most artificial light is around 400 to 600 lumens. If you go outside on a sunny day, the brightness ranges from about 1,000 lumens in the shade to more than 6,000 lumens on a large stretch of concrete, like a highway. A large snowfield on a bright day can reflect light at more than 12,000 lumens. Our eyes are comfortable until we get to around 3,500 lumens. Light over 10,000 lumens can result in temporary or even permanent blindness.
Light over 4,000 lumens appears as glare (white light)
See Understanding Ligt at HowStuffWorks.com

Polarization: Blocks glare from reflective surfaces. These lenses are ideal for water sports, bicycling, and driving.
Not available in some colors.
  Some places do not recommend polarized lenses for skiing, because you can't see ice. There seems to be an open debate on this issue.
  Another issue is the affect on LCD screens. I tried my Smith polarized glasses with my Palm PDA, Magellan GPS and Motorola Mobil Phone and although it did have some effect it wasn't significant. My Canon EOS DSLR camera screen blacked out when turned sideways (90°), but worked OK in normal (landscape) mode.
You can check for polarization by holding one pair of glasses in front of another and rotating one 90°; If they are polarized all light should be blocked.

A Dell LCD computer display, iPhone, iPod nano and touch and a gas station LCD blacked out with the sunglasses at a 45° angle (actually 39° according to one article) but was only slightly reduced otherwise.
There may also be issues with LCD screens on car dashboards, electronic games; I haven't tested them yet. See: Canteen Polarized Effects With LCD - Video at MetaCafe.com

Oakley says:

"Common techniques of incorporating polarization films into lenses by lamination (using adhesive to sandwich the film between lens layers) and drop forming (sandwiching then heating and pressing over contoured forms) can cause haziness and optical distortion. Oakley uses an injection-moulding process to infuse the lens material around the filter. This liquid fusion creates bonding at the molecular level."

Kaenon uses their SR-91 lens material for polarized lenses. It combines the crisp optical acuity of the finest glass lenses, in an ultra light weight, impact-proof lens. Kaenon Polarized SR-91 lenses have an optical acuity and clarity rating of "40" - the highest possible score in the stringent ANSI Z.87.1 testing regimen. They also have a Glare 86 Polarizing technology. See dpShop.ch

Photochromic: Applies to lenses that lighten or darken in response to the intensity of sunlight. However it doesn't work well in a car where the windshield blocks the UV and there is some reduction indoors. The Transitions™ lens designed to work behind the car windscreen has transmission levels of 35% - unactivated, 22% - in the car, 12% in direct sunlight.

Other Variable transmission techniques:
Oakley uses Iridium lens coatings to allow variable transmission of light from 9 to 92%.
Metallic oxides are fused to the lens at the molecular level, for optimal balance between reflection, transmission and absorption, depending on the targeted environment for usage.

Kaenon uses Light Transmission Control (LTC) technology.

Light Transmision Factor - LTF: Percent of light transmitted. General purpose sunglasses transmit from 8 to 40%.
Lenses with 85% LTF are used for fashion tints. Same as VLT. See Smith table below.

SPF (sun protection factor) Number representing the amount of sun something blocks. For example, if you normally begin to burn after half an hour in the sun, a sunscreen with an SPF of 2 should let you stay out twice as long (1 hour), SPF 4 should let you stay out four times as long, and so on.

Visible Light (Red, Orange, Yellow, Green, Blue, Violet) 500 THz - 900 THz / 400nm to 700nm.

VLT - Visable Light Transmission: See LTF.

Glossary of Eye Care Terms at AllAboutVision.com

An article at Science Museum of Minnesota's Science Buzz says:
You will not get tan as fast wearing sunglasses.
The pituitary gland is tied to your optic nerve and is sensitive to sunlight. When light enters your eyes, it triggers your pituitary gland to produce a melanocyte-stimulating hormone (MSH) that activates your melanocytes to produce melanin. Melanin is the material responsible for skin pigmentation, hair and eye color, so less melanin means less tanning and natural sun protection.

See: Eyeglasses for lens materials

Retailers:
Sunglass Hut
HeavyGlare
REI
Campmor

Lenscrafters and Pearl Vision offer a 30% discount for AAA

Manufacturers:
There must be a high profit margin because more and more companies are selling sunglasses. A few of the more popular ones are listed below.

What's Hot:
* Maui Jim
Kaenon Kore - Shades Of Light Transmission Can Be Selected
Oakley $70-205
  prescription lenses $245 incl polarization plus frame $150
* Smith $90-169
  prescription lenses $250 + $75 for porarized
* Bollé
Wiley X
Carrera
UVEX
In 1999 Bausch & Lomb sold their Ray Ban Sunglass line (Formed to make sunglasses for the Army Air Force in 1937) to the Italian Luxottica Group, but some lenses may still be made by B&L.
Wiley X and 7eye (PanOptx) make goggle like Sunglasses that has a foam eyecup which seals arount your eyes.
Flying Fisherman
Other brands listed at Consumer Guide to Sunglasses at eyeTopics.com

* Can get prescription lenses

I couldn't find any reviews. There are so many models and they change so much it would be difficult. Maui Jim and Kaenon were mentioned as high quality in a couple of forums.

Links:
Sunglass & Reader Division of the Vision Council
Glossary of Eye Care Terms at AllAboutVision.com
Sunglasses & the Health Effects of Solar UV Radiation at the Australian radiation protection and Nuclear Safety Agency (ARPANSA).
Sunglasses at BackpackGearTest
Sunglasses for Sports - Choosing the Optimal Lens Color at EyeTopics.com
How to Choose Sunglasses at REI
Sunglasses 101 at HeavyGlare.com
Choosing Ski Goggles and Eye wear at SimplyPiste.com
Lens Color Analysis at SportsVisions.com
Fishing Sunglasses at Technical-gear.com
Sunlight filters: eye protection and improvement of visual performance at INTERCAST EUROPE S.p.A
Sunglasses at HowStuffWorks.com
Tinted Eyeglass Lenses
Sun Protection in health.