SRB2 is a freeware Sonic game that I and a few others from IRC play. You can add several add-ons to the game and play Online with friends, and you can play Singleplayer and unlock several things ranging from cheats to bonus levels.
And just for the hell of it, here's some more copypasta from the features list on the site.
- Beautiful 3D Enviroment, with free movement in all directions.
- Multiple choice of characters, including Sonic, Tails and Knuckles, each with different moves, weaknesses and advantages.
- Customizable - includes the ability to create your own levels, characters, level music and sounds and, with knowledge of programming, even a whole new game.
- Internet multiplayer, with plenty of gameplay modes.
- Hidden levels and goodies to unlock.
- Original Soundtrack produced by some of the best Sonic music artists on the Internet.
- Over 30 levels to play, featuring a variety of locations, from green meadows, to deep underwater ruins, castles, deserts and space stations.
- Multiple routes to choose from.
- And much, much more!
That should cover just about everything.
