Press Up, Up, Down, Down, Left, Right, Left, Right, B, A during game play to spawn a medpak that will restore 25 points of health.
Restores ammo for current weapon:
Press Y, B, A, X, White, Y, X, A, B, White during game play.
Unlocks all chapters:
Press Left, Left, Left, Left, Black, Right, Right, Right, Right, White during game play.
Vogon Easter Egg:
In the Canals level, right before your battle with the hunter chopper there should be 3 sewege pipe openings. Hop into the one all the way on the left and follow it until you eventually find a vogon sitting around a fire melting marsh-mellows. When he sees you he'll get up and start spewing around 60 something lines of anticombine propaganda. After he's done he'll sit back down and offer you a marsh-mellow. Throwing it at him is very satisfying in a way!
(supplied by: sdemonik190)