XAMLPad is a tool for WPF that ships with the Windows SDK, and allows you to easily parse and render XAML snippets. As far as I know XAMLPad for Silverlight is not available as yet. However, Robby Ingebretsen who also works for IdentityMine has added Silverlight support to his Kaxaml tool. After you launch Kaxaml, you can open a new tab either in WPF, or in Silverlight.
By the way, this tool has been created in WPF. Really cool!