No, not alphabet soup... rather a set of cool apps that have a seamless installation experience and run in the browser. Secure, full power of the platform... i love it.

• The British Library has an amazing 3D book collection.
• Bascule did Doninoken, which is a cool demo (link from Karsten)
• In what is probably the single coolest usage I've seen (from a purely practical developer geek kind of way) XCEED creating an interactive XBAP to show off their new data grid.

Karen Corby, a fabulous PM on the Avalon team, wrote a great post about the security of XBAPs. Chris Sells and I got in a big debate about how much detail my book should go into this kind of detail. In the end I felt the deep dive into custom security policy was out of scope for a book like mine (more about the overall structure and conceptual model of the platform), but an API reference or deep dive book would be a better place for this.

While they don't appear to have a lot of content yet, XBAP.org looks like it might become an interesting place to search...