Unit Test Framework for Silverlight: UnitDriven
Some of my colleagues at Magenic, Justin Chase and Rocky Lhotka, have developed a very nifty framework for unit testing Silverlight apps - UnitDriven.
In particular, it is optimized for testing asynchronous code in Silverlight, which is virtually impossible with existing unit test frameworks - both from Microsoft and elsewhere. More details are available from the CodePlex site.