osu... that game in which you tap the different circles and draw lines in time to the music? If so, I say congrats! I saw one Osu! video, and it looked hard.
You tap the hit circles and follow the path of the sliders. If you're off the range, you lose some health.
It's Osu! Tatake! Ouendan! (japanese elite beat agents) The second is Osu! Tatake! Ouendan! 2 they are quite good. Many prefer them over EBA.
PC - Osu! (Website at http://osu.ppy.sh) DS - Osu! Tatakae! Ouendan and Moeru Nekketsu Rhythm Damashii! Osu Tatakae Ouendan 2