Sonic 2 HD is an unofficial HD reworking of the classic SEGA Genesis title, Sonic the Hedgehog 2. Of course, the name of the project more or less gives that away, but it's no less exciting. Headed up by the perpetually friendly, outgoing and emotionally stable folk at Sonic Retro, this game has a lot of fans fapping already.
According to the teaser site, big things are coming in twelve days. The project has been dark for quite some time, so we can expect some big reveals coming soon. Between this and Sonic Fan Remix, it seems that the unofficial portion of the Sonic franchise has a lot of awesomeness happening.
You may now proceed to comment about how "sad" it is that the fans are doing better than SEGA, as is the custom.