Open source software developer Libretro is bringing its popular emulator platform RetroArch to Steam on July 30. It's not every day that emulating software appears Valve's platform, and RetroArch is easily the most prominent to make the leap.
The gray legality of emulating suggests that Libretro would have trouble getting RetroArch on a closed platform like Steam, but as ArsTechnica points out, Valve doesn't appear to have any rules against emulators. In its announcement post on RetroArch's store page, Libretro is careful about how it's positioning the software.
"While there is nothing particularly [sic] about RetroArch or the Libretro API that has anything to do with emulators, most do it use it for this purpose," the post reads. The Steam version of RetroArch will be functionally identical to the one you can download right now (though it's only launching on Windows at first), but in future updates Libretro wants to "explore" leveraging Steam's functionality as a platform.
Libretro is also "open to dialogue with game developers/publishers that have the rights to original IP who want to bring their games over to Steam through the use of RetroArch."