Cheat Mode:
At the title screen before the opening FMV, press X, Y, Y, X, X, Y, Y and then start. This will then take you to the main screen.

During game pause and do the following:

Full Health:
Hold L+R and press Right on the D pad.

God Mode:
Hold L+R and press Left on the D Pad.

Level Skip:
Hold L+R and press Down on the D pad.

Full Pause Screen:
Press Start to pause game play, then hold X + Y.

Invincibility and One-hit Kills:
Press and hold the R + L Trigge. While holding these buttons press Right, Up, then start. You won't take damage and your instant kills will be worth 50,000 points!

Thanks to Revolution readers danny white and great saiyanman!