Elements of the source port of Doom integrated into the latest Garry's Mod look really great.

Shooting down (or running over) endless masses of imps and cacodemons is always fun, and doing it in a City 17-esque dystopian open environment is just makes it more special.

The mod's author claims once it is ready and bug free, which should be only in a few days, the mod will be available through the Steam Workshop.

gmDoom [YouTube]