During gameplay press ENTER to get the message box and put in a code below
then press ENTER again to activate: 

RESULT - CHEAT CODE
fastbuild: WarpTen
fastdeath: IocainePowder
godmode: WhosYourDaddy
Gold: KeyserSoze [amount](i.e. 'keysersoze 50000' will give you 50000 gold) Note - Default is 500.
Lumber: LeafItToMe [amount] Note - Default is 500.
resource: GreedIsGood [amount] Note - Default is 500.
food: PointBreak
mana: ThereIsNoSpoon
cooldown: TheDudeAbides
nodefeat: StrengthAndHonor
novictory: ItVexesMe
research: WhoIsJohnGalt
showmap: ISeeDeadPeople
techtree: Synergy
dawn: RiseAndShine
dusk: LightsOut
time of day: DaylightSavings [hour] (i.e. 'daylightsavings 12' will set time 
to 12 noon) Note - The cheat without any numbers halts day/night progression 
as a toggle.
upgrade: SharpAndShiny
level jump: Motherland [race] [level] (i.e. 'motherland human 04' will jump 
you to campaign level 3) N

Animal kill 
Keep clicking the mouse over the animals and they will eventually explode. 
Note: The explosion will not hurt anyone else. 

Ending bonus 
Successfully complete the game on the hard difficulty setting to see a 20 
second clip of StarCraft 2 running under the WarCraft 3 engine !! 

Find the enemy with out cheating
If you think you beat the level but it wont say it most likely there is one 
more thing out there. if you press Tab{on the key board}the map will turn 
black and the enemy will turn their color. Therefore it will be easy to spot 
the enemy.

Cheat submited by Jonas

Cheat Codes
While playing a game, 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. If you entered the code correctly, the message "Cheat enabled" will appear.

RESULT - CHEAT CODE
Invincibility and one-hit kills - whosyourdaddy
Infinite mana - thereisnospoon
500 gold and lumber - greedisgood
Continue playing after losing
in campaign mode - strengthandhonor
Full map - iseedeadpeople
Instant victory - allyourbasearebelongtous
Instant defeat - somebodysetusupthebomb

Another list
iseedeadpeople - Remove fog of war
allyourbasearebelongtous - Instant victory
somebodysetupusthebomb - Instant defeat
thereisnospoon - Unlimited mana
whosyourdaddy - God mode
strengthandhonor - Disable defeat conditions
itvexesme - Disable victory conditions
greedisgood - Receive 500 gold and lumber
greedisgood # - Receive # gold and lumber
keysersoze - Receive 500 gold
keysersoze # - Receive # gold
leafittome - Receive 500 lumber
leafittome # - Receive # lumber
pointbreak - Removes food limit
warpten - Speeds construction of buildings and units
iocainepowder - Fast death/decay
whoisjohngalt - Fast research
sharpandshiny - Research upgrades
synergy - Disable tech tree requirements
riseandshine - Set time to dawn
lightsout - Set time to dusk
daylightsavings - Set time of the day
daylightsavings - Toggle daylight progression
motherland [race] [level] - Level select
thedudeabides - Cool down

Starcraft Units
Finish the Night Elf last campaign in Hard mode to see a 20 seconds of Starcraft units in action based on Warcraft 3 engine. (NOTE: This is NOT a scene from Starcraft 2, it is just an example of what Starcraft units would look like on WC3's engine. Nothing more.)

Exploding Animals
To make one of the various animals in the game explode simply click on them a bunch of times and finally they will blow up in a large explosion.

Extra Credits
After completing the game on hard mode and normal mode, check the credits, you'll see a parade of units while the credits roll, ending in some hydralisks coming out and kill them (not the same as the marine/hydrlisk battle listed elsewhere here).

Humorous Sounds
Most of the units say something humorous if you click them multiple times.

EASTER EGGS
Most of the hidden easter eggs are scattered throughout the game. Use the "iseedeadpeople" cheat code to find them.

First Mission of the Prologue - Look at the island at the top of the screen to see some pirates guarding "phat lewt".

Undead Campaign: Chapter 1 - In the main town area after you get your shade, there are 2 civilians fighting with a large crowd watching. Named: Robert and Tyler (from the movie Fight Club).

Undead Campaign: Chapter 2 - After the cut scene in the middle where you find you need the urn, knock down the trees to the west of the green human base. When you bring a unit to the waterfall you see a flash of a baby panda sitting on the shoulders of another panda wearing a sword. It'll have the text written: "SECRET FOUND Panderan Relaxation Area"'

Knockdown the trees in the curve in the path which leads to the Rogue Wizard. A path to the left will lead to a cutscene with the Sasquatch and Elder Sasquatch. The elder Sasquatch leaves behind Claws of Attack +6.

Tichondrius is also on this map and will kill some humans.

Orc Campaign: Chapter 7 - Your tauren units will eventually run into a large salamander named 'Hungry Hungry Lizard,' a play on an old board game "Hungry Hungry Hippos." He'll eat his way through a large patch of mushrooms if you watch him long enough. When he's done eating, he will pop into a splash of green goo, revealing a path that leads to a nifty item, The Lion Horn of Stormwind, which generates a protective aura around your hero.

Night Elf Campaign: Chapter 2 - In the area where you get the first set of ballistas. Knock down the trees in a north easterly direction. Near the back, among the trees will be a hydralisk. Bring your hero to it to let it join your party.

Night Elf Campaign: Chapter 5 - In the cavern, try and go as far right as possible when you start out. Look for a bunch of mushrooms on the right side with a skull and bone in the ground in front of it. If you hear the priestess comments about the demon bile, you've gone too far. Use force of nature on the mushroom sto clear a path. You will see the path is littered with bones. When you bring units into the path you'll get another cut scene and The Largest Panda Ever (level 10) will come to attack you. It'll leave behind a Necklace of Spell Immunity.