In the game press [F10] and type in:

 BIGHEAD                Big head mode
 BIGSTICKOFDEATH        Get shotgun
 BURNYOURASSOFF         Get flamethrower
 COMBATISSCARY          Easy combat
 DEMONBEWARE            Get mercy bullets
 FLAMEONASTICK          Flaming ammo
 GETINTOMYBELLY         All weapons
 GIBNPLENTY             Gratuitous dismemberment
 GIVEMEFAITH            Restore health
 GOODTIMESMAN           Get dynamite
 HELLFREEZEOVER         Freeze enemies
 IAMAWIMPFORTHIS        100 bullets
 ICANSEE                Night vision goggles
 INSTANTCRASH           Crash the game
 IRULE                  Hard difficulty
 ISUCK                  Easy difficulty
 IWORKFORGOD            God mode
 MEDIUMRARE             Get crossbow
 MEETMYPALTOMMY         Get tommy gun
 NOD3D                  Invisibility
 PUZZLESARESCARY        Easy puzzles
 RECHARGE               Restore flashlight battery
 SMILEYNOMORE           Get elephant gun
 SNOWSTORM              Snowflakes fall
 SUNOFGOD               Get charge radiance emitter
 T2000                  Terminator skin
 THEDOGFARTED           Gas mask
 THUNDERSTORM           Create storm (even inside)
 VAMPBEWARE             Get lith bullets
 WWBEWARE               Get silver bullets
 BLANK                  Black, hot pink, and gray objects
 DARK                   Darker graphics
 TIME                   Slow or Speed up Time

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 In the demo version:

 In the game press [F10] and type in:

 GODGAMES               God mode
 MOREAMMO               More ammo
 GOLDMODE               Hidden message