While it might not be the first application to provide emulation on Steam it's probably one of the biggest projects as RetroArch is not a single emulator. Instead, it provides a tidy user interface to get lots of different emulators, as well as play media and all sorts. It's most well-known as a platform for emulators though.
After being available in a Steam Playtest for some time, today marks the official Steam release. Right now the amount of cores (what RetroArch refers to as emulators / games, done via their plugin system) available are limited but it's going to expand over time with each available as a Steam DLC.
To pre-empt the "but why on Steam, you could always get it from x/y/z" comment - the answer is simple. As with any Steam release, it puts a fresh set of eyes on it and opens it up to more people to download it and keep it up to date with the rest of your Steam library.
It seems one of the ways they've gotten around copyright / trademark issues is to not name the systems the cores on Steam emulate. You will find they're not directly named on any of the DLC pages. Here's the currently available cores to download on Steam as DLC:
- Final Burn Neo - a multi-arcade machine emulator
- Genesis Plus GX - Sega Genesis / Mega Drive, Sega/Mega CD, Master System, Game Gear, SG-1000 & Pico
- Mesen S - SNES
- Mesen - NES
- mGBA - GameBoy Advanced
- SameBoy - Game Boy, Game Boy Color and Super Game Boy
- Stella - Atari 2600
- Kronos - Sega Saturn
- Mupen64Plus-Next - Nintendo 64
- PCSX ReARMed - PlayStation
Find it now on Steam.
When the Steam Deck releases, this is going to help turn it into such an awesomely varied machine. I honestly cannot tell you how excited I am about having them together so easily.