A couple of posts back I blogged about a cool app that I have been working on called Sonic. Basically its a queryable media library that uses a semi custom IQueryProvider. It is also a good example of the MVVM pattern and custom templates in WPF.
I am totally happy with the results of this one, and I feel its a very cool article.
You can read more over at http://www.codeproject.com/KB/smart/Sonic.aspx