Gens is an open-source Sega Megadrive emulator. It works on Windows and needs Directx 7.0 or higher.
Genesis Plus features very accurate emulation of the original Sega Genesis, even to the point that some software which has problems on the real hardware (Sonic Crackers, for example) exhibits the same behaviour under emulation.
an enhanced port of Genesis Plus, an open-source Sega Mega Drive / Genesis emulator, now also emulating SG-1000, Master System, Game Gear and Sega/Mega CD hardware, running on Gamecube / Wii consoles.
Fusion is a Sega SG1000, SC3000, SF7000, Master System, Game Gear, Genesis/Megadrive, SVP, Pico, SegaCD/MegaCD and 32X emulator for Win9x/ME/2000/XP/Vista/Win7, Mac OSX(Intel), and Linux.