You Got Halflife 2 In My Doom, And Doom In My Halflife 2

We've seen film mashups, experienced amazing musical mashups from the likes of Ludachrist to Chrono Jigga, but we've never seen videogame mashups reach this level of amazing. A few passionate modders took it upon themselves to port Doom (GMDoom, or Garry's Mod Doom) into the Source engine, and the results are absolutely surreal/face meltingly amazing. The idea of fragging a few headcrabs with a BFG or that insanely satisfying shotgun from Doom II might just prompt a reinstall.