Osu!

osu! is a rhythm game made by Dean "Peppy" Herbert in 2007. It has gotten good reviews and has been recommended by professional gamers as a way of improving cursor aim and gaining advantage in other games. A repository of the game, mostly written in C#, can be found in GitHub. 
Most of the levels (beatmaps) are made by the community, which are made with the level creator that is in the game.
osu! has 4 game modes, osu!standard, osu!taiko, osu!catch and osu!mania. osu!standard is the most popular one.