Enter the following at the 'Cheats' screen:

EMERPUS    Invincibility 
FMNFB      All cheats shown + big head
TEXAS      All weapons for level 
MADMAN     Unlimited ammo
SELLOUT    Level select 
SLLEWGH    Invisibility 
HEID       Big heads
ZOO        Zoo mode (kill any animal with war club)
HUNTER     Unlock Demo mode and Target mini-game



Flying Raptor Glitch:
First, find a map with water in it that you can submerge in. For this 
glitch to work you must be a Raptor (you can choose either raptor you 
want,it won't matter). Find the body of water in the map and jump in 
it in 1st PERSON. Once you've submerged into the water, press X to 
switch the camera to 3rd PERSON. Hold A as if you're still swimming 
and *drum role* YOUR FLYING!!! You can now go anywhere on the map by 
just walking. If you want to go back on the ground ( and still want 
to live), simply switch into 1st PERSON ( press X ) and try your best 
to land back in water (instant death if you don't). Note: You are flying
because the game still thinks your in the water, that means you can be
drowned. You should get at least 2 Regen Power Ups ( the
purple looking clam you find throughout the map ).

Thanks to Revolution readers gabriel cardona, Black Jeebus, Fred Shaw, Chris, thestarfox64master and Dave!