Silverlight Spy is a development tool that allows you to poke around inside a running Silverlight 2 application. The interface is basically a browser view together with property inspectors for UI controls, events, XAML, and the like. Launching the application is run through a ClickOnce installer.
There are some rough spots, most noticeably having to do with interface responsiveness. Expanding part of the definition of a large application tends to freeze the interface for a while as the tree is enumerated. There's also currently only support for Silverlight 2. You won't be able to inspect into an existing Silverlight 1 application.