A cool little tool that comes in handy for UI development with WPF (Windows Presentation Foundation) is XamlPad.
When dealing with a large application, making small tweeks to UI can be time consuming, especially when you are not quite sure which tag or attribute to use. Compiling,running the application and then navigating to the correct part of the application to test a small change can take a long time.
Enter XamlPad. Its like notepad except it renders your XAML so you can see what your changes will do. It doesn't handle anything that needs to be compiled, such as code behind methods, but for layout it is brilliant.
There is also a button that enables the treeview of the XAML. I missed this on my first few uses of XAMPad but being able to see the tree that your XAML produces is great.