Enter ~ to bring down the console, then type one of the following codes:

Code - Effect

allammo - 999 for Weapons
behindview 1 - Third person view
behindview 0 - First person view
fov # - Changes field of view
ghost - Fly around and walk through walls
god - Invincibility
loaded - Gives you all the weapons
slomo # - Slowdown or speed up the time
suicide - Suicide
walk - Turns off ghost mode