Sega Mega CD
As the names suggest, it allows a Mega Drive to run compact discs, be it proprietary Mega-CD software, audio CDs or CD+Gdiscs. It also acts as an upgrade to the Mega Drive hardware, sporting an extra processor and extra RAM. The Mega-CD was first released in 1991 and was supported alongside regular Mega Drive cartridges.
For information, see Sega Mega-CD.