To use a cheat code, press the enter key, type in the code, and press enter again. The message "Cheat Code Enabled" should appear. Note that these codes only work in single-player missions and custom maps. 

WarpTen - Speeds construction of buildings and units

IocainePowder - Fast Death/Decay

WhosYourDaddy - God mode

KeyserSoze [amount] - Gives you X Gold (Note: in some missions you will have allies and team resources, using that code will also give the allies resources the amount you specify)

LeafitToMe [amount] - Gives you X Lumber (Note: in some missions you will have allies and team resources, using that code will also give the allies resources the amount you specify)

GreedIsGood [amount] - Gives you X Gold and Lumber (Note: in some missions you will have allies and team resources, using that code will also give the allies resources the amount you specify)

PointBreak - Removes food limit

ThereIsNoSpoon - Unlimited Mana

StrengthAndHonor - No defeat

Motherland [race] [level] - level jump

SomebodySetUpUsTheBomb - Instant defeat

AllYourBaseAreBelongToUs - Instant victory

WhoIsJohnGalt - Enable research

SharpAndShiny - Research upgrades

IseeDeadPeople - Remove fog of war

Synergy - Disable tech tree requirements

RiseAndShine - Set time of day to dawn

LightsOut - Set time of day to dusk

DayLightSavings [time] - If a time is specified, time of day is set to that, otherwise time of day is alternately halted/resumed

Another note, most of the cheats besides general ones will not work with your allies. i.e. if you use God mode cheat, it will only affect your team, and noone else.

-from FISKER_Q & poison
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Cheat mode:
Press [Enter] to display the two player message box, type one of the following codes, then press [Enter] again to activate the corresponding cheat function. The message "Cheat enabled" will appear to confirm correct code entry.

Full map - Iseedeadpeople 
Instant victory - allyourbasearebelongtous 
Instant defeat - somebodysetupusthebomb
Unlimited mana - thereisnospoon 
Invicible - whosyourdaddy
500 gold and 500 lumber - greedisgood 
Super Fast Build - warpten 
Die Really Fast - iocainepowder 
Get Gold: keysersoze [Amount] 
Get Lumber - leafittome [Amount] 
Cannot Win - itvexesme 
Automatic Daylight - riseandshine 
Automatic Night - lightsout 
Upgrade Everything 1 Level - sharpandshiny repeat this code to upgrade everything 1 level again 
Disable the Tech Tree - synergy