Sega Mega Modem
From Sega Retro
The Mega Modem is a modem for the Sega Mega Drive released in 1990 exclusively in Japan. It allows a Mega Drive owner to connect his console to the Internet through services provided by Sega. Ultimately, three services made use of the modem: the Mega Anser, Sega Meganet, and Sansan (there have been other banking services: Nagoya Home Banking and Osaka Bank My Line; and Sumisei Home Tanmatsu, which appears to be a life insurance system — whether or not they use Mega Anser is unconfirmed).
The modem connects to the back of a Mega Drive through the third DE-9 expansion port; consequently, it will only connect to a Model 1 Mega Drive. Furthermore, two models of the Mega Modem are known to exist: one that connects to the Internet through a phone line (which was sold both standalone and with the Meganet's Sega Game Library cartridge) and one that connects through an RS232C cable built into the unit.
According to promotional materials, the Mega Modem would have been renamed the TeleGenesis if it were released in the US.