Tafiti is one of the cooler (if not most useful) examples of Microsoft Silverlight 1.0. It's a search engine that allows you to search between media and regular results all while providing a fluid experience. They're now open sourcing it under the Microsoft Public License.
There are quite a few small Silverlight examples out there but I haven't seen code for anything large. Tafiti provides the source code for a meatier application as well as showing how to combine Silverlight and Live services (something I think will be very popular as Microsoft opens up their APIs. It also might be a good example of best practices for Silverlight, something that's just starting to come up.