GENS was the emulator I used a little over a decade ago. With the right additions and version, you could play Sega CD games on it with zero slowdown, even then. A good chunk of the Genesis games are part of the SEGA steam packs like Tanoomba said. I think one of the packages has like... almost all of the Genesis games we mentioned in this thread. Phantasy Stars 2-4, Shining Force 2, Beyond Oasis, Story of Thor, and a couple of others.
Oorrr they cost 2.99 each with the pack, or 61.99 for all of them as a bundle. I think I spent like 10 bucks on the package I got, and it has like 16 games or so. =/