While playing, press ~ to open the console, then type any of the following codes:

Code - Effect

cl_showinfo 1 - Show FPS
g_fly 1 - Fly Mode On
fly - Fly Mode
addwpn ? - List Weapons
addwpn ## - Add Weapon ## (from List)
suicide - Kill Self
map ## - Load Map ##
gotoroom room## - Select Room ##
g_noclip 1 - No Clipping Mode
game_godmode ## - God Mode (## = Current Health)