Genesis
AGES, Blades Of Vengeance, Consoles - Megadrive, DCGen, DGen, DGen/SDL, FreeRoms: Genesis, Gametown, Genecyst, Generator
Master System and Game Gear
AmiMasterGear, Calypso, Chasms, DumpMasters, Emukon, Massacre Homepage, Massage, MasterGear, MesaDX, Osmose
Kega
A freeware 32X, Game Gear, Master System, Mega Drive / Genesis, SC3000, Sega CD / Mega CD, and SG1000 emulator for Windows, by KGen author Steve Snake.
SegaEMU
Freeware emulator for Windows. An emulator project that hopes to one day emulate all existing sega systems.