Sonic Robo Blast 2 is a free 3D Sonic the Hedgehog fan-game built on a heavily modified fork of the Doom Legacy source port of Doom. It is closely inspired by the original Sonic games from the Sega Genesis, and attempts to recreate their classic design in 3D with robust multiplayer and mod support.
Seriously entertaining game, played through it mutiable times and have shared it with tons of my firends who have all loved it. It's got a very replayable singleplayer mode and a very fun multiplayer.