Not one I've covered before: Pegasus Frontend, a free and open source (GPL) game launcher that seems pretty sweet has Linux support.
Pegasus is a graphical frontend for browsing your game library (especially retro games) and launching them from one place. It's focusing on customizability, cross platform support (including embedded devices) and high performance.
The focus on customization is certainly interesting, giving you the ability to completely the change the way it looks. It's even compatible with EmulationStation's gamelist files, which is an older emulator front-end which no longer seems to be maintained.
I've been playing with it for a while and I quite like it, it's still rough around the edges as it seems to be reasonably early in development. Things like metadata for some games just randomly not appearing, GOG support on Linux isn't ideal (although it works) and so on.
Pegasus Frontend just recently had another release with Alpha 12. It has builds available for Linux, Raspberry Pi, Android, macOS and Windows.