libretro

Frontends

※ Both Retroarch and Ludo have full OS versions you can install on a single-board computer.

Retroarch - Shit UI/UX, but supports the most cores and features. Has ports for all sorts of systems.
Ludo - Better UX and similar UI to Retroarch. Less support for extras, but easier to just play shit?
Minir - More traditional emulator UI.
Phoenix - frontend and Lutris-like library manager.