From Sega Retro
PlaySEGA is an official Sega website which features a variety of flash games and an in-browser Mega Drive emulator. Guests are free to play some flash games, however you need to be a registered user to play Mega Drive games or flash games with user-created content. "VIP" users (i.e. those paying a subscription fee) can create their own levels for a special version of Sonic the Hedgehog, and also receive a PlaySEGA USB joypad.
Once registered, users can play Mega Drive games through a browser-based version of jEnesis. Available games are as follows:
This list intends to include all flash games which were once available at PlaySEGA. Some of them may not be available at the service anymore.
Subscribers, or "VIP" members, are given access to level editors for Sonic the Hedgehog Level Creator (a special flash version of Sonic the Hedgehog) and Stuntmaster Motocross 2. They are also given a free PlaySEGA USB joypad contingent on subscribing for at least 3 months, which costs £12.99 or US$14.95. However, there is a shipping fee. Additional joypads may be purchased. 
Signing up for a free 5-day trial gives you temporary access to the level editors. Anyone may play levels created by VIP members whether they are subscribers or not, but you will need to register an account (which is free).