Wicked cool application at http://www.bl.uk/. The application allows you look at two notebooks from Leonardo da Vinci (the original stuff!), plus a few other very interesting, precious books.
All powered by WPF – note that the application is an .xbap, which downloads without prompting and runs in a very safe sandbox.
Hats off to the British Library for creating the application and making it (together with the great content) available to the public, and to the WPF team for providing the platform to deliver this.