I was using the XamlPad from the .NET SDK, so I did a quick search for something similar but with more features and utility.
My search turned up multiple alternatives, so I was wondering if anyone had any specific recommendations on which XamlPad alternative to choose.
As others have said, Kaxaml* is a great alternative to XamlPad. Kaxaml is great for quickly testing out layouts. And has some great snippets that show how to style basic UIElements and Brushes.
However, Kaxaml looses the ability to render the layout as soon as you start getting into !OOB namespaces.
IIRC, VS2010 has much better support for XAML. Can't wait :)
* link is to the GitHub project; the project's .com website is now being squatted and is NSFW.