XamlPadX
I always thought the original XamlPad was a cool tool, at the time I was using it WPF was still in beta and Visual Studio 2005 didn't really have designer support. I lost track of the tool sometime after .NET 3.0 was released, and honestly I don't know if its still available if you download the Windows SDK. All I know is that a full install of Visual Studio 2008 Team System Edition does not appear to have it. Well it turns out someone else has created a similar tool (named XamlPadX) and has even made it to version 4 without me noticing (XamlPadX 4.0).