Cardboard tube Reaver
Pause game play as Kain, then press Up, Down, Left, Right, R2, L2, Triangle, Down, Circle. Your Reaver will be replaced with a cardboard tube (as from a paper towel roll).

Switching Reaver Alignment
If you switch alignments while holding down the square button when your reaver energy is full, you can sucessfully change which spell you use without losing your full bar.

Unlock everything
Just beat the game and save the info to your memory card then go to bonus materials to see them. Don't worry about finding the tomes, they are all opened when you beat the game:

Kain's tomes - found throughout Kain's journey
Making of LOK Defiance - finish game and save to memory card
Raziel's tomes - found throughout Raziel's journey
Soul Reaver 1 - opening start new game
Soul Reaver 2 - opening start new game

Disable Textures
Pause the game and press L, Down, Black, Right, Black, Up, Y, L, Down.

Wireframe Mode
Pause the game and press L, Down, L, Up, R, White, L, Down, Y.

Cartoon Mode
Pause the game and press Up, Down, Up, Down, R, Black, Down, B, Y.

All Slams
Pause the game and press Right, Down, Up, Down(2), R, Y, B, Down.

All Dark Chronicles
Pause the game and press R, Down, Black, L, Right, Black, Y, Down, L.

All Power-ups
Pause the game and press Left(2), Up(2), L, Black, B, Down, Y.

Invincibility
Pause the game and press Up, Down, Right, Down, R, Black, Down, Y, L.

Refill Health
Pause the game and press Left, Right, Left, Right, R, L, B, Y, Down.