Well I found it hard the other day to try and debug the XAML Parser. I was having an issue in Silverlight 2 and databinding. I came accross several posts and have heard from Microsoft that we cannot debug the XAML Parser. I needed to see the values of what was rendered XAML.
It almost seemed impossible until I came accross a great tool by First Floor Software called Silverlight Spy.
All I can say is that this tool saved me and is something that will remain in my toolkit forever:
Fully functional for thrity days and less than 200$, well worth it.