The code entry dialog is accessed by pressing CTRL-ALT-X
Then enter the following cheats...

cruise control          turns on cruise control
im back                 repairs any damage
lock and load           refills ammo
mr fabulous             makes the user invulnerable
moon                    Moon gravity
mars                    Mars gravity
earth                   Earth gravity
jupiter                 Jupiter gravity
sampo                   $999,999
beefcake beefcake       Fill weapons and ammo

Daniel Ankrum, Mario Friedmann and Craig Tabita