Cheat mode:
Type aiculedssul at the copyright screen, then press [F9] at the Acclaim logo.

Fight Jade:
Win one round using only low kicks in the match before the "?" round.

Fight Noob Saibot:
Win 25 consecutive matches, then win the first round of the next match.

Fight Smoke:
Quickly press [F1] or [F2] when Dan Forden appears and says "Toasty!" on the Portal stage.

Unlimited credits:
Wait until the countdown at the continue screen begins. Press [F2] to start a two player game, then press [F1] while in the two player menu. After you defeat the player two you will continue the game at the place you left off without losing a credit.