Start the game with the "+set console 1" command line parameter, then press ~ to display the console window and type "cheats on". Now, you can enter the following codes:
God mode: god
Additional health: health [100 or more]
Increase power: boost power
Increase attack: boost attack
Increase speed: boost speed
Increase acro: boost acro
Increase vitality: boost vitality
Increase all stats: boost all
Weapon select: weapon_give_[1-10]
Infinite ammunition: g_unlimited_ammo 1
Kill all monsters: massacre
Invisibility: notarget
Rampage mode: rampage
No clipping mode: noclip
Screenshot: screenshot
Disable cheats: cheats 0