Sega Meganet was a network service in Japan for use with the Sega Mega Drive. To use the network service, a Sega Mega Modem needed to be attached to the Mega Drive and a subscription fee of ¥800 per month for 6 months needed to be paid. It could then be used to go online to play games with modem support with other players in Japan. There was also a special cartridge, called Sega Game Toshokan, which allowed people who had the cartridge inserted to download and play a small variety of games that were never commercially released in Japan.
List of Retail Games With Online Play Support