Gum Application (binaries): files.flatredball.com/content/Tools/Gum/Gum.zip
Gum Source Code: https://www.github.com/vchelaru/Gum
Note that Gum uses XNA so the XNA runtime must be installed here:
Gum is a general-purpose UI tool for games, mockups, and websites. Gum saves to an XML format which you can include in your projects, but also supports a plugin system (using MEF) for custom exports.
Gum combines the power of programming concepts such as inheritance, instancing, and default/overriding variable values with a easy-to-use visual editor.
These images were taken from the free sample project here: http://download-codeplex.sec.s-msft.com/Download?ProjectName=gum\&DownloadId=1426617