SMS
Did anybody ever actually own a Master System besides me? It's the one system that I could never find used games for at any video games store (well, only once). What's too bad is that it never became more popular than it did, because it had some nice games out for it, along with some nice peripherals, like the 3D shutter glasses. Most people thought they were gimmicky, but I liked them and they added depth to the games that they supported.

Which emulator you use depends on which OS you use. If you are on Windows 2000/XP, then go for Dega, currently the most user friendly Windows emulator for the SMS, programmed by Dave, original author of DTMNT, DGen, Final Burn, and general Sega freak.
But MEKA is truly outstanding, and award winner for most beautiful GUI in an emulator. The emulator is free, but if you register (either with an original SMS cartridge or a small donation), you get new releases in advance of the general public. (Yes, I'm a registered user).

You can also find support in MESS, GenS, and Kega.

Phantasy Star Zillion Alex Kidd in Miracle World