Play as Earth Worm Jim:
At character select screen hold L button and press B, C-Left, C-Up, C-Right, C-Down, C-Up. Now go to (?) and press R button.

Play as Booger Man:
At character select screen hold L button and press B, B, C-Right, C-Right, C-Left, C-Right. Now go to (?) and press R button.

Play as High Five:
At character select screen hold L button and press C-Up, C-Down, C-Left, C-Right, B, A. Now go to (?) and press R button.

Play as Sumo Santa:
At character select screen hold L button and press A, C-Right, A, C-Right, C-Down, C-Up. Now at the (?) box press R until you find him.

Taunt:
Hold Z and push any kick or punch button.

Debug mode:
At character select screen hold L button and press A, B, A, B, A, B, C-Left, C-Right, Up, C-Right, C-left, C-right. When fighters appear press Start to pause and quit game. Now you will be sent to the debug menu.

Thanks to Revolution readers Jeff, StarWolf60, Jack, D.L.Dumas, Robbie, and Justin Green!