GENECYST
Current Version: Genecyst .32

Genecyst is a fantastic Genesis EMU from Bloodlust Software.
This has become a very popular emulator thanks to its speed, smooth graphics, sound, and a high ROM compatibility.
Homepage

GENEM
Current Versions:
PC - Genem .19/Gennew .19
MacOS - 0.20.2

Genem was virtually the only Genesis emulator for a while.
This is a great emulator with sound support and a high ROM compatibility.
Unfortunately, Genem PC is rumored to be on hold. :(
Genem PC currently has no homepage, but the emu can be downloaded from here.
Mac version available here: MacOS Genem Hompage

KGEN
Current Versions:
KGEN v0.34b

KGen has reached Genecyst proportions! Simply amazing!
High ROM compatibility, Good Speed, and Great sound.
Homepage

Magnum Opus
Current Version: None

New Genesis Emulator in the works.
The aim is to create a Sega Genesis
/ Mega Drive emulator that runs at a fast frame rate on a 486 WITH sound emulation too.
Homepage

EMX
Current Version: None

EmX is a Genesis emulator being developed by Teegosoft.
It is planned to be a multisystem emulator.
Homepage

32XULATOR
Current Version: None

Canceled

NextLEVEL
SegaCD Emulator
Current Version: Beta

A new emulator being developed by Christian Fowler.
If you have any Sega CD documents, please send them to him.
Homepage

W95 GenEMU
Current Version: None

New Windows 95 Genesis emulator being developed by Jelloboy.