I know this topic's been done a few times before, so I'm gonna repeat myself too, just everything from Painkiller, the electrodriver is great, sure, but so is the grenade launching stake gun, I mean you can fire a grenade up, fire a stake at the grenade and it'll (I assume) puncture the grenade and keep going, almost turning the grenade into a rocket. Then there's the weapon in the usual FPS 'Fist' slot, not only is it a lawnmower, its also a grappling hook and a laser trap. Everything in that game was so inventive, considering you only had like 5 actual weapons, there was about 20 ways to kill people.
Honestly, its dirt cheap on Steam, I think, go try it if you like FPS games!