Resturant	15-20%
Buffet	5-10%
Bartender	$1-2 a round -or- 15% to 20% of the tab, with a minimum of 50 cents per soft drink, $1 per alcoholic drink
Cocktail Waitresses	$1-2 per drink
Take-out Delivery	10-15%
Catering	20%
Floral deliveries	$1-3 Complicated or large plants up to $10
Hair stylist	15-20%
Hotel housekeeper	$2-5/night - left daily in a sealed envelope.
Valet	$2-5
SkyCap/BellHop	$2 first bag, $1 per additional bag
Concierge	$5 for tickets or reservations, $10 if hard to get; no need to tip for answering questions
Taxi driver	15-20%

Links:
Tip20! - Tipping Information, advice, standards & practices - Also has International Standards
General Tipping Guidelines at EmilyPost.com