The problem with the WarCraft 3 cheat codes is that the computer also gets some of them and in some cases it can take better advantage of them than you can. For example, the basic WarpTen to speed up your building is great, but it also speeds up the computer's ability to build and it can figure out where it wants buildings much faster than you can. This code ends up making it much harder for us humans. There are a few that can really make a difference (besides god mode which just makes it too easy.)
Code Purpose Both Sides Advantage
WarpTen Speeds construction of buildings and units Yes No
IocainePowder Fast Death/Decay ? ?
WhosYourDaddy God mode No Too Big (No challenge anymore)
KeyserSoze (amount) Gives you X Gold Yes No (It usually hurts since the computer can expand very fast)
LeafitToMe (amount) Gives you X Lumber ? ?
GreedIsGood (amount) Gives you X Gold and Lumber Yes No (But it makes the game less boaring)
PointBreak Removes food limit Probably Yes

You can build lots of barracks and build huge armies if you start building the units before you hit the 90 limit. The computer won't do that.

ThereIsNoSpoon Unlimited Mana ? ?
StrengthAndHonor No defeat No Not Really
Motherland (race) (level) level jump N/A N/A
SomebodySetUpUsTheBomb Instant defeat N/A No
AllYourBaseAreBelongToUs Instant victory N/A Not really, it ends the match
WhoIsJohnGalt Fast research ? ?
SharpAndShiny Research upgrades Yes Usually not
IseeDeadPeople Remove fog of war Apparently Not Yes. The computer seems to know all about the map so this just evens things out.
Synergy Disable tech tree requirements Yes No
RiseAndShine Set time of day to dawn Yes Depends on who you are.
LightsOut Set time of day to dusk Yes Depends on who you are.
DayLightSavings (time) If a time is specified, time of day is set to that, otherwise time of day is alternately halted/resumed Yes Depends on who you are.