During gameplay, press pause and enter a code.

Invisibility:
Hold L and press Up, Left, Right, Up, B, A.

All weapons:
Hold L and press Right, B, B, Y, B, A.

Slow motion:
Hold L and press Left, Down, Up, Right, B, A.

Fish-eye camera:
Hold L and press Left, Up, Right, Down, B, A.

Thanks to Revolution reader Gunny_Hartman!