It's called osu!, one of those rythm-based games (love 'em).
It's based off the games for the DS: "Osu! Tatakae Ouendan" and "Elite Beat Agents".
The PC game is 100% free, includes a solo or multiplayer games, with online stat tracking and rankings on the leaderboards and the in-game chat. You can also spectate other players and watch replays.
Gameplay video:
http://youtube.com/watch?v=JZ_NKPgGo9Q
Main page:
http://osu.ppy.sh/
FAQ (more rules here... too much to explain ^.^ ):
http://osu.ppy.sh/index.php?p=faq
No own screenshots, sorry. ^^;
Hope to see more of you ingame
