Sega SG-1000: Available in limited markets
Sega SG-1000 II: Updated version of the SG-1000, includes a keyboard
Sega Mark III: Available in limited markets (Japan/Australia/New Zealand)
Sega SC-3000: A computer version of the SG-1000
Sega SC-3000H: An updated version with more RAM and keyboard (the original keyboard was of the low-end membrane type).
Sega Master System: Essentially the same as the Mark III only with a different name and a few minor adjustments
Sega Master System II: An update to the Master System with a new smaller redesigned frame, as well as inbuilt game "Alex the Kidd in Miracle World.
Sega Mega Drive: Known as the Sega Genesis in North America due to another company owning the Mega Drive trademark in that region.
Sega Mega-CD: Known simply as the Sega CD for the North American market, it allowed CD based games as well as Audio CDs to be played on the Mega Drive.
Sega 32X: Hardware update to the Mega Drive allowing 32-bit based games to be played
Sega Multi-Mega: a portable CD player with the functionalities of a Sega Mega Drive and Sega Mega CD.
Sega TeraDrive: A 16-bit PC with an integrated Mega Drive. The system was only released in Japan.
Sega Neptune: A Sega Mega Drive/32X hybrid. It never passed the prototype stage. Only two empty cases are known to exist.
Sega Saturn: Sega's other 32-bit console released before the 32X in Japan, but after the 32X in North America and Europe.
Sega Dreamcast: First 128-bit (sixth generation) console, also Sega's last console.
Sega Pico: an educational gaming system.
Sega Advanced Pico Beena: Successor of the Sega Pico
Amstrad Mega PC: Although not actually produced by Sega themselves, the Mega PC is Amstrad's version of the TeraDrive for European and Australian markets.