What is "osu!"

osu! is a freeware rhythm game developed by Dean Herbert, originally for Microsoft Windows. It is written in C# on the .NET Framework. The game has also been ported to OS X, iOS, Android, and Windows Phone. Its gameplay is based on commercial titles including Osu! Tatakae! Ouendan, Elite Beat Agents, Taiko no Tatsujin, beatmania IIDX, O2Jam, and DJMax.