Osu! A game kinda like Guitar Hero, only with less flailing about, more mouse action, and you don't have to play with a guitar.
Most of the stuff there is animu songs too, so you'll be fine even if you're not into western songs.
Here. Obviously, that's one of the harder maps, on Insane difficulty, so here is an easier one. The site's here, for anyone who wants to download it. Just download, download some beatmaps from the same site and play.