Mr Toots - a very funny, very deadly weapon - unlocked on completion.
You can also unlock Mr Toots on your first playthrough, in-game. When taking part in the Marauder Defenses mission, having used the turret to defend against the mass of enemies, ignore the request to Meet with Kara. While looking over the turret towards the force field, turn to the left. Move towards the ammo crate and you should see a door in the darkness to the right. Blow it up and blast your way into the corridor with a big fall down. Once at the bottom, blow all the partition walls ahead, until you get to the 'floor' area. A floor will appear as you walk forwards. Blow the door at the end to enter Cabbit Season. You can then pick up Mr Toots, and use it for the rest of your playthrough.
Cheats Menu (see Cheats section) - unlocked on completion - does not affect achievements.
New Game (+) - this mode allows you to play through keeping all your weapons and Forge abilities from the previous playthrough.