INIshell is a graphical user interface for numerical simulation software. It dynamically builds GUIs from XML files containing semantic descriptions of the models' parameters and allows for easy rapid deployment of new software features in the GUI.
An open-source GUI prototyping tool that's available for ALL platforms.
Pencil is built for the purpose of providing a free and open-source GUI prototyping tool that people can easily install and use to create mockups in popular desktop platforms.