Clayfighter 63 1/3

On the character select screen, you can do the following:

Play as Sumo Santa and Dr. Kiln
Sumo Santa: Hold L and press A, botttom c, right c, top c, left c, and b
Dr. Kiln: Hold L and press B, left c, top c, right c, bottom c, and A.

Hidden Options
Hold L, press top c, right c, left c, bottom c, b and a.

Taunts
Press simultaneously L, R, and B or A

Play as Boogerman
Hold l, press on the joypad up, right, down, left, right

Random Character Selection
Hold Down R and L