Oh man, you HAVE to check out SRB2 (Sonic Robo Blast 2) The name may seem like a stupid fan creation, but it is AWESOME!
It's built off the Doom Legacy engine, and has BEAUTIFUL levels, that make any modern 3D sonic game look like shit. The platforming is playble, you can speedrun levels, and it feels like a Genesis Sonic game was ported to PC.
The multiplayer is AMAZING too! It's full of classic playmodes like CTF, Deathmatch/TDM, Tag, Hide and seek, and older versions have things like race through special race levels.
There is also a rich modding community with playermodels, levels, and new content out the ass.
SRB2.org
Check it out, it's IMO the best thing on the Doom engine, and is worth a HEARTY play.