Name: OSU!
Website: https://osu.ppy.sh/
Why: It's the bestest (and probably only) free-to-win rhythm game. Popular, crazy and fun. A must-have. The con is that it's dependend on .NET CORE 3.1 SDK. You won't regret after a match or two.
License: MIT
Download: https://github.com/ppy/osu
Source Code: https://github.com/ppy/osu