Invisibility: hold L1+R1 and press Square, Square, Circle, Circle, Triangle, X
Invincibility: hold L1+R1 and press Triangle, Triangle, X, X, Square, Circle
All power-ups: hold L2+R2 and press Triangle, Circle, Square, X, Triangle, X
All Inventory: hold L2+R2 and press X, Square, Circle, Triangle, Square, Circle
Reset Physical: hold L1+R1 and press X, Circle, Triangle, Square, X, Circle
Reset Magic: hold L1+R1 and press Triangle, Circle, X, Square, Triangle, Circle
Level skip: hold L1+R1+L2+R2 and press Triangle, X, Square, Circle, Circle, Circle 

Interview  -  If you put the game in a cd player and listen to tract number 15. It's an excerpt from the Todd McFarlane (the creator of Spawn) interview on one of the Playstation Underground CDs. 

Magic Attacks  -  Don't let your magic meter get too low while using these. If you do you'll die.

FireBall: Back, Back, Down, Down, X
Magical Blast: Back, Forward, Square
Hell Gauntlet: Back, Forward, Triangle
Ice Blast: Forward, Forward, Down, Down, Circle 

Play as Overkill  -  This is a difficult trick to pull off. First get up to Overkill (the first one), then fight him until he falls. But when he is falling, before he hits the ground, hit start then hold L1+R2 and press triangle square square O X X triangle. Then when you press start spawns cape will wrap around him. Then when his cape comes off SPAWN will be OVERKILL. He can punch, kick, and shoot a laser from his eye, so have fun. 

Refill Health  -  To refill your health, while playing press L1 + L2. This only works a limited number of times. 

Special Attacks 

Right Cross: Forward, Forward, Triangle
Elbow Smash: Forward, Forward, Square
Right Power Kick: Forward, Forward, Circle
Left Power Kick: Forward, Forward, X
Cape/Chain Attack Right: Half-circle back to front + Circle
Cape/Chain Attack Left: Half-circle back to front + Square
Arm Rip: Half-circle front to back + X