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.
This game is such a jewel. A must play game for all Doom and Sonic fans!