osu! is a mouse-based rhythm game that's pretty big (and can be lots of fun), too.
Osu! looks pretty intense and fun. I wonder how well it works on a touchscreen device like a Surface.
It'd certainly play very well if it works well. Looks like it's based directly on the Ouendan series (Elite Beat Agents) for the Nintendo DS where the games are played with a stylus.
I didn't know about osu! but I love Elite Beat Agents so I will definitely be checking this out!
You should check out Stepmania:
http://stepmaniaonline.net/index.php?page=downloads
It's a free, multiplatform DDR work-alike that has keyboard song packs as well as pad song packs (you can map keys for the non-keyboard song packs too).