Gul 2.0 is a powerfull and free Xul parser/interpreter library for Phpgtk 2 written in php. Gul supports PHP and basic CSS features into Xul. Gulpad example provides a split screen where the Xul code can be typed and immediately previewed.
InterPython (stands Python InterOperability) is a .NET library that allows a python interpreter to be "embedded" in your .NET programs, and gives you (almost) transparent access to python libraries and services in .NET programs. Great for GUIs, etc.