Here are some Game Genie codes that i found interesting:

SNES MK1

656A-47AF
Start the game under 2 players. Make player 2 either Scorpion or Sub. Beat up the first player with player 2 and dont continue. Now you will be fighting Reptile, but press start on player one's controller. Now player 2 will be Reptile with whoever's moves you selected. This will also turn anyone else you select green with the name Reptile in the energy bar.

Genesis MK2

??MT-BE76 (replace the ?? with the following to tourn player 2 into)

B4-Smoke
CC-Jade
B8-Noob
BW-Kintaro
BO-Shao Kahn

With Kintaro and Shao Kahn, C will be the only hitting button to use. Pressing LK with any of the above characters will freexe the game. Do not try to use the spear with Smoke or Noob either.

If anyone else has any kool Game Genie codes then please post, and also post your replys on my codes.