GenEm is a Sega Mega Drive / Genesis emulator for MS-DOS and Windows 9X programmed in 1996 and 1997 by Markus Gietzen. It's the second Genesis emulator released (the first one being Mega Drive), and as such, has speed issues.
The DOS Version was the first emulator to feature (prelimilary) sound emulation, although the YM2612 isn't exactly emulated. The emulator tries (and 95% of the times fails) to convert the voices, commands, etc. to the format of the OPL3-chip used on various Sound Blaster cards. If you have any other sound card, it won't give you FM sound. Additionally, although Z80 emulation is present, players must activate it with a switch in command prompt -z80, or for fake emulation, -fakez80. Games which require Z80 emulation to work properly (for example Sonic 2 or newer, which have the sound driver on Z80, or DAC samples in most games) must use the -z80 switch. The DAC emulation is, however, extremely buggy and slow.
It comes with 2 executables.
The Windows version seems to lack sound emulation, even though there is an option to enable the Z80. It does not time refreshes properly and consequently will run too fast on newer computers.