Sorry for the pun, but Wildpalm have done it again. Not content with forcing a ZX Spectrum and a Nintendo Gameboy into Series 60, they've now launched Sonic Boy, a Sega Game Gear and Sega Master System emulator. Well worth checking out if you have some legal ROMS.
Get Your Games In N-Gear
Of course you could pay for the Emu. Or you download SMSplus (its freeware) for S60 and you can play MS/GG Games on your Phone without having to pay for the emu !