After the beginning animation it says "Choose Campaign". On this 
screen type:

unlockcheatmode (if it worked you'll hear a clicking sound)

Then the following can be typed from within the game or at any 
menu screen. Again after each entry you'll hear a click sound:

zombie -- after being killed, type this in and you'll come back 
as a zombie. Your little head shot will still show a skull.
bighead -- it'll give you a BIG HEAD!
killthemall -- I believe this kills all the enemies but not your 
good guys. (translation, kill them all, NOT kill the mall ;)
showtheend --show the end of the game.
missiondone -- will successfully complete your current mission, 
regardless of where you and your troops are.
missionfail -- fail your current mission (why would you want to 
do that?)
openalldoor -- open all doors.
goodhealth -- I didn't understand this one. I'd type it and 
nothing ever happend.
allammo -- THE BEST CHEAT! Gives you ALL the guns, rocket 
launchers, grenades, etc... 
laracroft -- you become a guy in drag that's trying to look like 
her... funny! (I think this gives you a few different uniform 
choices as well).
enemylookf -- gives you a view of the enemy.
enemylookb -- same exept behind you. 
playercoords -- displays your coordinates in upper left corner.
noplayerhits -- again, I'd type this in, yet I'd take hits and 
die. But the Zombie thing would usually bring you back for 
more.