At the cheats menu, type in these codes:

LLSNMRTN - Invincibility
JFFSPNGDNBRG - Infinite lives
MGRLSGTM - All weapons
RHNSRLL - Unlimited ammo
TRNTNNQ - All maps
DGHTTSRS - All keys
TKMDKK - Show enemies
PTLFGNDS - Warp to level 1
QTMBGS - Warp to level 2
GTMNDSBF - Warp to level 3
NCPGHM - Warp to level 4
RSTPDFRLP - Warp to level 5
NBCD - Warp to level 7
LFRRSPR - Warp to level 8
CRCDND - Longhunter warp
CTNTSCND - Mantis warp
THSFNDNT - Trex warp
HSTSMN - Campaigner warp
HTSDNM - Gallery
LRNMSNHR - Credits
MBRNKLSN - Sprit mode
HFLLTHSH - Tiny enemies
PRMSHN - Big heads
BGGNTSS - Disco mode
KNTSFSKS - Fly mode
GSRCGKFS - Quack mode
CRLSFNDNGS - New color