Mortal Kombat Trilogy

Harder Game:  For a greater challenge in one player mode, highlight Kano,
then press Down + Start. You will hear Shao Kahn say,"You'll never win."
When you start a new tournament, there will be plenty of Endurance matches
to go around. 

Level Select: Highlight Sonya and press Up + Start.

More Options:  During the story line, quickly press Left, C, Up, C, Right,
C, Down, C, L, R, R, R, L, L. If you were quick enough, a tone will sound
and two new selections will appearon the Options screen. 
Play as Human Smoke: Choose Smoke then press and hold Away, High Punch,
High Kick, L, R before the match starts. 
Play as Motaro: This trick only works on the Desert and Wastelands. 
Choose anyone, but before the match starts press and hold Away + Low Kick
+ High Kick. When the match starts you will morph into Motaro. 
Unlimited Credits:  For unlimited credits, press Down, Down, Up, Up,
Right, Right, Left, Left at the attract mode screen.