The Sega Game Toshokan (lit. "Sega Game Library") was a short-lived service (1990-1992) on which subscribers could download Mega Drive titles through Mega Modem. It was essentially a predecessor to Sega Channel.
After the service's closure, some of its exclusive titles were re-released by Sega in the Game no Kanzume series of compilations on Sega CD, as well as a Japan-only service in 2004.
(Source: Sega Retro, Sonic Retro)