Official Sega site - so you don't have to dig out those suspiciously acquired roms to enjoy a few hours play.
In beta at the moment so it might be a site for the future.
I'm not too keen on their subscription costs, but free for 5 days seems a winner to me!
From their site:
PlaySEGA is a new online gaming service from SEGA with a host of free online games and Mega Drive classics including complete versions of Sonic the Hedgehog, Golden Axe, Streets of Rage, Ice Shuffle, Altered Beast, Ecco the Dolphin, Virtua Fighter 2 and more.
Create your own PlaySEGA account to save and load your favourite games, collect PlaySEGA Rings to trade for account accessories and entry into weekly cash draws. PlaySEGA offers a VIP subscription service as well as free online games,
PlaySEGA VIP gives you access to more exclusive games and the chance to design your own game with Sonic the Hedgehog and Stuntmaster Motorcross Level Creators.
You can also earn double PlaySEGA Rings and enjoy continuous gameplay without advert interruption. Sign-up is quick and simple and includes a limited edition PlaySEGA Joypad with subscriptions over 3 months while stocks last.