Command & Conquer
Sandbagging trick:

Use sandbags to build all the way to your enemies base.
Now build some defenses, guns or whatever to take them out!
A ton of turrents is especcially effective!

Change Screen:

Hold L and press C-Up or C-Down to zoom or un-zoom the screen

Economics 101:

Build a Tiberium Refinery. Sell the actual refinery and you will
receive $1,000 credits. You end up with an ore truck. You save
$200 credits. If you were to build and ore truck through a war
factory, it would cost $1,200 credits

Economics 201:

Need an Orca but don't have enough money? Instead of
buying it, simply build another heli-pad... and then scrap it.
You will be able to keep the Orca that came with it and save
450 credits!

Sell Men:

Place your Mini-Gunners really close to a sand bag wall and
sell the wall. If your men were close enough, and it works right,
your men have been sold.

Sell vehicles:

Place a not wanted vehicle on a repair bay and put the sell
cursor over the vehicle and preess "A".

Be able To Build Opponent's Units:

Get your engineer to take over a enemies construction yard
and you can build their units.

APC Trick:

If you're playing as GDI, and you have access to APCs and
Engineers, then you can try this trick. Build 2 APCs, and put
them each to a C-button. Now build 10 engineers, split hem
into 2 groups of 5, put each group on a C-button, and load
each group onto an APC. Now, drive the APCs over to the
enemy base, and park them next to the NOD construction
yard/barracks/airstrip. If the APCs are blown up, it's okay, the
engineers will walk out still alive. Send all the engineers into
NOD buildings, going into the construction yard first. Now,
whenever you blow up a NOD building, they won't be able to
replace it. And, if you captured their Hand of Nod or airstrip,
then they won't be able to build any more units if you blow
them all up.

1500 gold cheat:

message: To get 1500 tiberian, set up 5 grenaders and
5 minigunners beside the tiberian factory.  Wait till an enemy
comes and start attacking the factory.  In like 2 seconds your
money will raise up by 1500, and your factory will still have full
energy if you do this correctly.

Hidden Level:

On the GDI mission on the Czech Republic: Level 6. 
When your playing as the Commando guy, blow up all the
sam sites in the beginning.  Take the chopper to the far left
and as quickly as you can, release the commando.  If done
correctly, it will say your mission is a failure.  Restart mission
and you will end up in a hidden level.

Obelisk tower of light tip:

If you can't get past your enemys obelisk tower of lights
(laser tower) just take out their power plants and the laser
towers will be disabled.

Harvester tip:

If you are sick of getting killed by enemy forces when you try
to destroy their harvester then attack the harvester with air
craft.(Nod-Attack helicopter) (GDI-Orca.) Then your enemy will
not attack you.