Drive Or Die Expert multi-player level
At the level selection screen, highlight the multi-player Drive Or Die Expert level, then hold L + R for about three seconds to unlock the level.

Helter Skelter multi-player level
Play 20 or more multi-player games in one sitting to unlock the Helter Skelter multi-player level. When this is done, force the game to save by changing your options or weapons configuration in single player mode to make this level always available.

Extra multi-player maps
Get an "A" grade on single player missions to unlock additional multi-player maps.

Bonuses
Successfully complete the challenge games to get bonus items such as a starting drone with 9999 bullets, etc.

Hidden artwork
Place the game disc in a PC compatible CD-ROM drive to find artwork from the game