It seems after years of Sega pissing away the Sonic franchise some fans have taken it upon themselves to make there own Sonic game, and the results look pretty sweet. Now I haven't played it for myself but judging by the video the looks like a definite step in the right direction. Then again I would have said that about Sonic 4 until I played the demo.
But why take my word for it, judge for yourself.
BTW you can download the fan made game for free, here.