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