Why does this thread not exist already?
RetroArch does a really great job integrating with the API for RetroAchievements, which for those that don't know, adds achievements to your retro games much like modern systems do. There is also a Leaderboard, which is a pretty cool hook that allows you to compete against others in the community. It's pretty baller.
In addition to the officially supported systems, RetroArch also supports cheevos for Atari 2600 and FBA. I hear they're adding PS1 as well! The list of supported systems is pretty big, and I'm pretty sure there's others that RetroArch supports that don't have a RetroAchievements equivalent.
There's a RetroArch bounty for improving the pop-ups to make them look more like a modern console (right now it uses the in-built message display), so it's good to know that's gonna happen.
Well long story short, I like to see what others are up to. So if you aren't in there, get in there! If you are in there, add me to your friend list!
http://retroachievements.org/user/Adultery
Learn how to configure RetroArch and which cores support it here:
https://docs.libretro.com/guides/retroachievements/
Read a brief overview about it here:
http://www.retroarch.com/index.php?page=achievements
If you want to join me in supporting the bounty for pop-up beautification, you can do that here (until they finally put it on bountysource):
https://github.com/libretro/RetroArch/issues/3946#issuecomment-379507788
Trust me, it's worth the small bit of work it takes to get this rolling.