I might be more excited about this than I was about the Web Application Projects preview we just released...
One of the last things I worked on in the ASP.NET runtime test team was validating a version of the sources of our providers so that users could download them and see first hand what our design practices are. That work is finally availible to you.
I spend a large portion of my blogging efforts and forum responses trying to promote good designs or revealing little snippets of how we implemented the StaticSiteMapProvider and the XmlSiteMapProvider. Now, the sources for these providers are availible for download all ASP.NET users and users can see examples of well written providers or copy the code to help bootstrap their own projects.
Download here: http://download.microsoft.com/download/a/b/3/ab3c284b-dc9a-473d-b7e3-33bacfcc8e98/ProviderToolkitSamples.msi
More Info here: http://msdn.microsoft.com/asp.net/downloads/providers/default.aspx
One note: Functionally these providers are nearly equivelent to those actually in the framework, but because they occasionally used some internal APIs, we had to alter a few calls to use public methods instead as well as some other minor changes.