If you visit one of the sites and you find an interactive website pages with xbap extensions ( such as : home.xbap) those files are .net pages with WPF browser template project available in .Net 3.5.
Don't think its a silverlight pages or any other technology has been used,it's pure WPF browser project in .net 3.5,and all created pages take xbap extension,those files have the power of .net framework in your browser,it's tested on IE and FireFox,I have test it also on the last version of Firefox 3.0.4,it works fine.
If you would like to see a sample of simple interactive page using WPF browser application,here it is :
If you are familiar with ClickOnce Deployment in Windows Applications in .Net,it's the same but on your browser :).
It's really a great step in the web development by using the full power of .Net functionality and deployment mechanism on your favorite web browser.
If you want the sample code for the posted sample,it is here :