Check out Sonic Robo Blast 2, a 3D open-source Sonic the Hedgehog fangame built from Doom

A reader recently let us know of Sonic Robo Blast 2, a 3D free and open source sonic game built from the open source Doom Legacy game engine. SRB2 is closely inspired by the original Sonic games from the Sega Genesis, and attempts to recreate the design in 3D. While SRB2 isn't fully completed, it already features tons of levels, enemies, speed, and quite a lot of the fun that the original Sonic games provided.