Press N for 100 credits
Press V for noclip
Press P to get all weapons (plus cheat weapons, lasers and such)
Press C to have a copy of yourself flail in ragdoll mode
Press Z to have everything slow to a crawl for half a second or so
Press Q to reload the first level "City"
Press Y to have it thunder in the background
Press I to toggle the shaders on/off
Press X to see the bones of the ragdolls