In no particular oder:
Portal Gun (Portal & Portal 2)
Sheep / Super Sheep (Worms series)
Canon (Serious Sam series)
Laser Gun (Serious Sam TFE & TSE)
Minigun (every game except Saints Row 2, there it feels more like a bigger SMG)
Your Eternal Reward (Team Fortress 2)
Sentry (Team Fortress 2 and Worms 4 Mayhem / Worms Ultimate Mayhem)
Planet Buster (Spore)
Gravity Gun (Half Life 2)
Machete (Postal 2) it's a machete that you throw like a boomerang, always comes back, instant kill for almost every enemy. Badass.
Too bad it kinda makes the game crash...