Pause the game, hold L + R and enter the following codes:

Restore Health:
Y, Down, A, Up.

Restore Ammo:
A, Down, Y, Up.

Gain 1000 XP:
A, Down, Down, Down.

Level 2 skills:
B, Right, B, Right.

Level 4 skills:
Y, Up, Y, Up.

Level 6 skills:
X, Left, X, Left.

Level 8 skills:
A, A, Down, Down.

The following codes will only work after the game has been completed.

Invincibility:
Y, X, A, B.

Infinite missile weapons:
X, B, A, Y.

Devastating attacks:
X, X, B, B.

All combo upgrades:
Y, B, Y, B.

Small enemies:
Y, Y, A, A.

Slow motion:
Y, B, A, X.

Thanks to Revolution readers Michael DeLucia, billy, tj, Hawkeye, Alan Alvarez, Jarad "Kanadwen" Morris, Jordan Hess, joe klinect, alpal4000, GandalftheWhite, Craig Brazier, Brian Tanabe, Sean Cornell and Garrick Silvers!