A couple of programmers just outdid the entire team at Sega. It’s pretty incredible what a man and his computer can do. Almost a 1Up to the TM Sonic 4, I can’t imagine this download will last for long. If you’re an old fan of the Genesis games, you should check out their site.
The plan is to make their physics engine open source so anybody can modify it. Legally, I can see a problem spreading, but morally, you have to applaud such hard effort.