I've recently been more and more interested in Web 2.0 frameworks that support widgets.  Silverlight has an excellent mashup protocol but many clients aren't always ready to jump to that type of framework unless they are doing a good deal of video processing.  Dropthings is a widget framework portal that has an accompanying book that describes the framework, is optimized for performance, and utilizes a number of the newer Microsoft technologies.  It's worth checking out.

Dropthings - Free AJAX Web Portal, Web 2.0 Start Page built on ASP.NET 3.5