RGtk2 is a binding for R to the GTK2 library and dependent libraries. You can use it to construct arbitrarily complex GUI's from R. Your GUI can be integrated with other GTK GUI's such as that of GGobi via rggobi. It works on all three major platforms (Linux, Mac, and Windows).
Open-jACOB is a free framework for building a new generation of richer, more interactive, database driven, highly personalized cross-browser web applications. The M$ Access replacement for sophisticated Web Applications
The Pencil Project is a free and open source tool for making diagrams and GUI prototyping that everyone can use. Web designers and web application developers can use Pencil Project creating mockups for their websites or web applications easily.