osu! is an amazing rhythm game. Try osu!lazer, it’s the new client: https://osu.ppy.sh/home/download
I also like Pioneers and Endless Sky, both space sims.
This might not count, but both Lichess, a chess website, and OGS, a Go website are open source.
There is also Mindustry, but I haven’t played it.
Slowly first, then all at once.