Osu

Osu is a korean game similar to Guitar Hero but with the mouse. Many league players play Osu because it helps you get better at lasthitting (doing the last bit of damage to an enemy minion which gives you a bit of gold). Since lasthitting is such an important mechanic and Osu can help improve this.

In Osu, you can download a song from the site with a beatmap made by the community, or make your own. To play you just have to hit the circles that appear on the screen, the better you time the clicks, the more points you get. Osu Site