EmuNations

Gearsystem is a very accurate, cross-platform Sega Master System / Game Gear / SG-1000 emulator written in C++ that runs on Windows, macOS, Linux, BSD and RetroArch. The main focus is readability of source code, very high compatibility and quality of life features.

Multiple Arcade Machine Emulator is a free and open source emulator designed to recreate the hardware of a wide variety of (mostly vintage) computers, video game consoles and calculators. 

The aim of MAME is to be a reference to the inner workings of the emulated machines; the ability to actually play the games is considered "a nice side effect". The source code to MAME serves as this documentation. The emulator now supports over seven thousand unique games and ten thousand actual ROM image sets, though not all of the supported games are playable. It worth mentioning that the sister-project MESS was integrated into MAME.

Unofficial recent MAME 32-bit versions are available here..

Genesis Plus GX core:

RetroArch is a frontend for libretro emulator cores, game engines and even media players. It enables you to run classic games using Libretro on a wide range of computers and consoles through its unified graphical interface. Settings are also unified so configuration can be done once and applied and applied to all. It has advanced features like shaders, netplay, rewinding, next-frame response times, and more.

It can run on the usual platforms like Windows, Mac OS X and Linux, but it stands alone in that it can support far more platforms beyond just that.

• Ludo - A smaller libretro front end by only implementing the core features and by targeting less platforms but definitely shares a lot of the same core values.

• Lakka - lightweight Linux distribution for small computers like Raspberry Pi

• RArch - It incorporates Kaillera to offer the possibility to play games online. 

BizHawk is a multi-platform emulator written in C# which focuses on core accuracy and power user tools. Its main features are TASVideo (Tool-Assisted Speedrunners) for full recording and debugging tools as well as Lua scripting. The consoles covered include but are not limited to Apple II, Atari 2600/7800, ColecoVision, DOS, Game Boy/Game Boy Color/Super Game Boy, MSX, Nintendo Entertainement System, Super Nintendo, Gameboy/Color, Sega Master, Sega Genesis, Sega 32X, Sega Saturn, Sega GameGear,  ZX Spectrum, Atari 2600/7800, Commodore 64 and a few more!

TwoMbit is a cycle/bus accurate emulator for Sega's 8-bit game consoles (Master System, Gamegear, SG-1000)