The early/mid 90s was a confusing time to love Sega games. Sure, they were still pumping out classic software left, right and centre, but they couldn't make their mind up about which piece of home console hardware they wanted you to own the most. The Mega CD was a CD based expansion for their trusty Mega Drive, which let you play many of the same games as you could play on cartridges, just with higher audio fidelity and price tags. There was also a few Mega CD exclusive games though, probably most noteworthy was their flagship platforming mascot's imaginatively titled Sonic CD, which is now heading to PC (again).
Yes, there was a PC port of this back in 1996, but wikipedia tells me that it won't work on modern operating systems, and Sega's PC ports back then were only OK at best anyway. This newly announced downloadable version, like many ports of old Sega stuff to new platforms, started off life as a new fan made engine, that's ended up getting official support, leading to a commercial release. New features include widescreen support and acheivements, although no price or release date has been set as of yet.
I never owned a Mega CD, so I'm not too familiar with this entry in the series, but I know many regard it as the series high point.
View this trailer to revisit a time when Sonic was a respectable game series, rather than the complete and utter joke we're left with today.
And here's the lovely looking intro video. How come the new 3D sonic games look nothing like this?
More details over at the Sega blog.