TO unlock the Level select cheat, type "jumplevel name", using "oasis", "anubis", "ragnarok", "juggernaut", and "dead" as level names. Do that while in the game.
Use the following codes while in the game:
macleod: invincibility
keymaster: get armor, health, ammo
dirtyharry: get infinite ammo
buzz: glider wings
framerate: shows framerate
phantom: press F11 and F12 to spy
gohome: reset at starting position
thrasher: shows all enemies on radar
snipsnip: character's sex
timewarp: timer
bigears: sounds
showall: full map
weaponcam: camera follows ammo
singletrac: "Singletrac Rules!" message