osu!, going under the current development name of osu!lazer is a very popular free rhythm game and they're now doing official builds for Linux gamers.
It's actually inspired by an older game called Osu! Tatakae! Ouendan, which was released in 2005 for the Nintendo DS. osu! was originally only available for Windows, then ported to macOS and eventually they started work on osu!lazer as an open source remake of the original client to eventually replace it. There's been various unofficial builds out there, since it's open source and up on GitHub but they're now making Linux a bit more official.
With the 2020.221.0 release, they provided an AppImage to hopefully enable osu! to be played across many distributions with ease. Keep in mind since this is the first attempt it's still in testing, with no auto-updates just yet but they're working on it which is awesome.
How popular is it? Amazingly so! This is what their official stats said earlier:
15,210,892 registered players, 18,526 currently online in 663 games