Spoiler:
Null Driver. You have to unlock everything to get it, and that means beat the game with no damage suffered, beat the game with 100 crack successes after subtracting fails, beat the game on Ultimortal, Expert, Hard, and Normal, and collect all the posters. Maybe collect the ribbons as well. I say "maybe" because I downloaded a save with the Null Driver unlocked, and there's no way to tell if the ribbons have been collected.

After obtaining the Null Driver, it stays in your inventory until your turn off the game. Even if you quit to the title screen and load a new file, it will stay in your inventory. When you fire the Null Driver, you get 99 stat points, 99 of every ammo type, you kill every non-boss enemy on the screen with an explosion (this can damage you as well if you're too close), and fucks with all the graphical data. The more you fire it, the more you lose the ability to tell what the physical properties of the area around you is. Otherwise it's an awesome gun.