Cheat mode:
Enter the codes during gameplay:

Result - Code:

God mode - zingermans
All weapons - eatangelos
All keys - currygoat
All keys - oralgroove or algroove
Level warp - whammazoom
Bouncing weapons - ericaanne
All accessories - alifalafel
Fish-eye view - bittersweet
View John's head/Shrunken window - pigfarmer
Psycho Guide-Bot - gowingnut
Crash into robots with one hit - godzilla
Level select (1-24) - freespace
Homing weapons - lpnlizard
Invincibility - almighty
Creates/Replaces the Guide-Bot - helpvishnu
Full Map - rockrgrl
Display framerate - frametime
Mechs destroy each other - silkwing
1% shields and energy - gabbagabbahey
Wowie Zowie weapons - motherlode
Wowie Zowie weapons - honestbob
Full map1 - joshuaakira
Bouncing weapons - duddaboo
Underwater appearing walls - bittersweet
Shield at 27% - blueorb
Rapid fire - wildfire
Bosses do not shoot - imagespace