Enter the following codes at the Main Menu. You'll hear a sound to confirm:

Enable Despair        Right, Right, Left, Left, Right, Left, Right 

Enable Bunny Despair  Right, Right, Left, Left, Right, Left, Right, C-Left, C-Up, C-Right, C-Down 

Titan mode            Right, Right, Left, Left, Right, Left, Right, C-Up, C-Down, C-Up, C-Down

Fight Gabrielle       Right, Right, Left,Left, Right, Left, Right, C-Left, C-Left, C-Left, C-Left 

During play, hold A and enter the following codes:

Small Fighters        Right, Right, Left, Left, Right, Left, Right, WP, SP, SK, WK, Target

Invincible            Right, Right, Left, Left, Right, Left, Right, SK, SK, SK, WK, Crouch

Slippery Stage        Right, Right, Left, left, Right, Left, Right, WP, WP, WP, WK, WK, WK

Take No Damage        Right, Right, Left, Left, Right, Left, Right, SP, SP, Sp, WK, WK, WK, A

Take Less Damage      Right, Right, Left, Left, Right, Left, Right, SP, SP, SP, WK, WK, WK, Z

One Hit Kill          Right, Right, Left, Left, Right, Left, Right, SP, SP, WK, WK, WK, R

Slow Mode             Right, Right, Left, Left, Right, Left, Right, SP, SP, Z 

Big Head Mode         Right, Right, Left, Left, Right, Left, Right, Z 

Big Feet Mode         Right, Right, Left, Left, Right, Left, Right, R
 

Thanks to Revolution readers Chris Lee, julian, Tim "the code finder" Terveer and Tompkins!