Pause the game, hold L+R and enter the following codes. You'll hear a sound to confirm:

Maximum health 
Right, B, Down, Up, Down, Up 

Upgrade health to next level 
Right, A, Left, Y, Up, Down 

All abilities 
Up, Up, Down, Right, Right, Left, B, Right, Left, Down 

Pass through barriers 
Down, B, B, Left, Right, Y, Up 

Wall climbing 
Y, Down, L2, Right, Up, Down 

Refill magic 
Right, Right, Left, Y, Right, Down 

Maximum magic 
Y, Right, Down, Right, Up, Y, Left 

Soul Reaver 
Down, Y, L2, Right(2), Down(2), Left, Y, Down, Right 

Aerial Reaver 
A, Right, Up(2), Y, Left(2), Right, Up 

Kain Reaver 
A, B, Right, Y, Left(2), Right, Up 

Fire Reaver 
Down, Up, Right, Up, Down, Left, B, Right, Down 

Shift at any time 
Up, Up, Down, Right, Right, Left, B, Right, Left, Down 

Water Glyph 
Down, B, Up, Down, Right 

Fire Glyph 
Up, Up, Right, Up, Y, L2, Right 

Stone Glyph 
Down, B, Up, Left, Down, Right, Right 

Force Glyph 
Down, Left, Y, Down, Up 

Sunlight Glyph: 
Hold L then press Left, B, Left, Right, Right, Up, Up, Left

Thanks to Revolution readers Ariel Kattan and Prince Bagdasarian!