I don't know what it's actual name is, but I've affectionately dubbed it the "Portal Spawner," which is a gun from Half-Life: Opposing Force.
It does pretty much like it sounds: It creates portals. Not like the Portal gun; this one creates unstable portals and shoots them at foes to randomly suck them into dimensions (and not in one piece). The secondary fire drags you through a stable portal into the alien dimension to grab ammo, armor, and health.
Lets see... Other than that, I would say the Gatling Laser from Fallout 3, because, well, its a fucking gatling laser, and given how badly the rest of the laser weapons sucked in that game, I was impressed that it was flat-out awesome, and by far the best gun in the game (only because the mini-nuke launcher has much more limited ammo and will get you killed in close quarters).