All characters:
Pause, hold L and press X, B, Left, Y, Right, Right.

Play as Tek-9:
Beat the game with any character to unlock the Tek-9 glider.

Play as Hex:
Beat the game with all three main characters (Sid, Zero-G, and Chops) to unlock Hex as a playable character.

Play as Synder:
Beat Time Challenge Mode in all 8 levels.

Thanks to Revolution readers Pogo and Timothy Lynum!