Osu – open-source, extensible rhythm game (github.com)

That's super cool, I've known about OSU for years and had no idea the whole thing was Open Source. I highly recommend anyone who has inclination towards rhythm games who hasn't heard of it to try it out. Very unique rhythm experience, multiple modes, lots of well made free community maps. My only big advice when looking through community maps is to favor ones made in the last 5 years or so, some of the very old maps are a little, stiff, for lack of a better word.