I know many people are looking for FireBug type tool (which is available as plug-in in Mozilla Firefox for editing HTML runtime in any running Web application) for WPF or Silverlight. I can recommend Snoop tool that is very similar to Firebug.
Here is the URL:
http://snoopwpf.codeplex.com/[
^].
This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.
A list of licenses authors might use can be found here