The Yosuga no Sora OST is lovely: http://www.youtube.com/watch?v=5A72McHhJkM
What are some OSTs you've enjoyed?
I liked a couple tracks on the KLK OST but didn't find that great for listening to by itself (although I think it's used very well in the series itself).
I downloaded the Penguindrum music collection from BakaBT and there are some really great tracks in there.
Azumanga Daioh's OST
Muromi-san's Character CD is especially good.