Well, we have a touhou topic on the osu! boards; might as well have an osu! topic on the touhou boards, neh?
http://osu.ppy.shosu! is a freeware PC version of a popular series of DS games, Osu! Tatakae! Ouendan! and Elite Beat Agents. Using mouse or tablet is a bit of a higher learning curve than a stylus and touchscreen, but since all the maps are user-made, most of them are generally possible for both.
I know at least three other people on these boards who play (if only because they have their bars on their signatures), and I know I've told at least a couple of people about who I am in the game.
I've really gotten rusty since I stopped playing frequently about half a year ago. Beatmaps that I used to clear no problem with S ranks now have a few misses in them, and usually end up with B ranks.
Gaah the top score is in the 4 billions, I don't even have 1 billion...