Open Source User Interface (UI) Software

Code::Blocks is a free, open-source, cross-platform C, C++ and Fortran IDE built to meet the most demanding needs of its users. It is designed to be very extensible and fully configurable. Finally, an IDE with all the features you need, having a consistent look, feel and operation across platforms. Built around a plugin framework, Code::Blocks can be extended with plugins. Any kind of functionality can be added by installing/coding a plugin. For instance, compiling and debugging functionality is already provided by plugins! We hope you enjoy using Code::Blocks! The Code::Blocks Team

Pyjamas is a python to Javascript compiler, Widget set, Framework and Toolkit for Application development that runs on Web browsers. The developer need not know anything about AJAX: all the AJAX tricks, for all major browsers, are entirely taken care of.

A cross-platform GUI, new language (eC), and 3D engine.

JAXX is a powerful XML user interface language for Java. JAXX source files compile into ordinary Java classes, making integration a snap. JAXX features advanced data binding and event handling, and uses (fully compiled) Java as its scripting language

Kopi provides a development environment for interactive database applications using Java, JDBC and Swing. It extends the Java language with money, date and time types, adds operator overloading and allows seamless integration of SQL.

rngcc is "really nice GNU C compiler" it's the GUI for the standard GCC on Unix systems rngcc is fully written in c

Brainfuck Studio is an IDE for the Brainfuck programming language consisting of a text editor, a compiler as well as a debugger.