If your a Sonic fan, this is a pretty cool game you should try out
Game Download - http://www.srb2.org/
Addons - http://mb.srb2.org/forumdisplay.php?f=79
As of right now the game isn't complete yet, but there are lots of stuff you can do.
Co-op, Match, Team Match, CTF, Race, Tag, Hide and Seek. (All online of course)
There is also addons to the game such as :
Co-op, Match, and CTF stages, characters, crazy lua stuff, and mods.
Plus the game is open source.
But... this game only has one real flaw...It has a god awful netcode
For the most part though it's only a problem with a server with alot of people.
And with all that said... Give it a try!