Press F10 and type one of the following codes:
Invincibility: iworkforgod
Invisibility: nod3d
Crossbow: mediumrare
Shotgun: bigstickofdeath
Tommygun: meetmypaltommy
Elephant gun: smileynomore
Flaming ammunition: flameonastick
Charge radiance emitter: sunofgod
Night vision goggles: icansee
Gas mask: thedogfarted
Full health: givemefaith
Disable enemy AI: hellfreezeover
Easy combat: combatisscary
Easy difficulty: isuck
Flamethrower: burnyourassoff
Dynamite: goodtimesman
Hard difficulty: irule
Dismemberment mode: gibnplenty
Big head mode: bighead
Terminator 2 skin texture: t2000
Weather is raining: thunderstorm
Weather is snowing: snowstorm
Slow down or speed up time: time
Black, hot pink, and gray objects: blank
All weapons: getintomybelly
100 bullets: iamawimpforthis
Silver bullets: wwbeware
Lith bullets: vampbeware
Mercy bullets: demonbeware
Easy puzzles: puzzlesarescary
Darker graphics: dark
Darker color scheme: itisdarknow
Crash game: instantcrash