RetroArch is a frontend for emulators, game engines and media players. It enables you to run classic games using Libretro cores 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.
Consoles covered include but are not limited to PlayStation 1, Super Nintendo, Nintendo Entertainment System, GameBoy/GameBoy Color, GameBoy Advance, Arcade, Neo Geo Pocket Color, Virtual Boy, Sega Genesis/Mega Drive, Sega Master System/Sega Game Gear, Sega CD/Mega CD, PC Engine/PC Engine CD, WonderSwan Color/Crystal, Cave Story, Doom(s) and much 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
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!
Linux: As stated by the authors, support is a work-in-progress
Multiple Arcade Machine Emulator is a free and open source emulator designed to recreate the hardware of arcade game systems. The aim of MAME is to be a reference to the inner workings of the emulated arcade machines; the ability to actually play the games is considered "a nice side effect". 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 MESS was integrated upstream into MAME in the past.
Unofficial recent MAME 32-bit versions are available here..