Press Up, Up, Down, Down, Left, Right, Left, Right, B, A during game play to spawn a medpak that restores 25 health points.
Press Y, B, A, X, White, Y, X, A, B, White during gameplay to restore your current weapon's ammunition.
Unlock all chapters:
Press Left, Left, Left, Left, Black, Right, Right, Right, Right, White during gameplay to unlock all the chapters.
Headcrabs roasting on a open fire:
After you blow up the chopper, make for the gate. Inside there, use the boat as a booster to jump into the higher tunnel. Inside it, you'll see a pool of nuclear waste. Swim to the other side of it, and head down the tunnel on the left. You'll find a alien slave cooking headcrabs on a stick! Not too useful, but funny.
Easy Queen Antlion kill
Take a crossbow and aim at the middle of the antlion queen and shoot about 4 times and it should come crashing down
Thanks to Revolution readers Seth, One_of_Them, and nick!