Turning the Pages 2.0 by Armadillo Systems uses WPF3D to allow you to manipulate books in 3D space and collaborate with other readers. The British Library is using it for a few of the books in their remarkable collection. Thanks to this app, two of Leonardo da Vinci's codices have been brought together for the first time!
By the way, don't you just love XBAPs? Just one click and you're using 3D in your browser.