Kega Fusion is a multi-console emulator encoded by Steve Snake, which will emulate most Sega consoles, including the Game Gear, Genesis/Mega Drive, Master System, Sega32X, Sega CD, SG-1000, and SG-3000. The best Sega emulator overall.
MAME (Multiple Arcade Machine Emulator) is an emulator designed to recreate the hardware of coin-operated arcade game systems.
MAME Plus! is a popular unofficial version based on the original MAME which boasts extra features and games above and beyond the official version. It also supports high score saving, unlike the regular versions of MAME.
Gens is a Sega Genesis/Mega Drive, Sega CD, and 32X Emulator for the Windows platform. It has many nice, exclusive features including a multi language menu, and netplay with Kaillera - allowing for online SOR gaming!
This sucker can literally do ANYTHING to a Genesis/Mega Drive ROM, including patch IPS files.
Lunar IPS is basically a Windows-based version of Segatool. It is intended as an easy to use, lightweight IPS patch utility for Windows. It can both create and apply IPS patches.