Invincibility:
In the Cheat menu type Left, , Up, , Right, , Down, .

Invisibility:
In the cheat menu type Left, , Right, , Left, , Right, .

Slow-Motion:
In the cheat menu type Right, , Up, , Right, , Up,

Unlimited Ammunition:
While in the game type Down, , Left, , Down, , Left, .

Infinite Ammo:
While in the game press pause and type Down, , Left, O, Down, , Left, O.

Xen Gravity:
While in the game press pause and type Up, , Down, , Up, , Down, .

Alien Mode:
While in the game press pause and type Up, , Up, , Up, , Up, .

Thanks to Revolution reader MudDustKnot!