Ones I have personally used and endorse (will play most games at full speed with sufficient hardware) will be marked by an * (asterisk). For more emulators and systems go to: my main thread
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*Gens: A good emulator that is compatible with most games.
*GensPlus!: Based on the Gens emulator, this has support for joystick axes, cheat files, more resolutions, new sounds, graphic filters, and adds in Game Gear + Master System emulation with some optimizations.
*Gens32: Like GensPlus! above, this is based on the Gens emulator, adding in more options.
*Kega Fusion: like Dgen, Kega Fusion focuses on accuracy instead of speed and miscellaneous features. Is able to maintain good speed and compatibility with most games for Sega SG1000, SC3000, Master System, Game Gear, Genesis/Megadrive, SegaCD/MegaCD and 32X.
-Dgen: An older emulator with good compatibility on lower-spec hardware. This project was abandoned sometime before 2005, with 1.17 being the last version released in 2001. This was picked up by Tamentis in 2008 and is an okay choice for people with low-spec machines.
-GenesisPlus: focuses on accuracy over speed. A great choice if you want compatibility and have the hardware to run it.
-Gens/GS: Is also based on Gens, and is trying to clean up the main source code, while adding in features from the other 'branches'.
-PicoDrive: This emulator is made to be widely available on different systems, and has support for Sega Pico drives too.
-Regen: like DGen and Kega Fusion, this focuses on accuracy over speed, and has compatibility with most games.