GUI

FOX GUI Toolkit - An open source cross-platform C++ based Toolkit for GUI development.

GUIS - Open source widget server. (Python, Ruby, GTK2) [OS Independent]

InputGUIDO - A GUI being developed for older DOS systems that are incapable of running Windows.

Java Gui Builder - An engine that reads XML files describing graphical user interfaces layout.

KronoJunior - Open source GUI for less powerful or old computers.

ObjectCentral - V - A free, multiple platform C++ graphical user interface framework.

The Official FVWM Homepage - The standard window manager for linux, stable but not as pretty as Window Maker or Enlightenment.

Prima - Toolkit for multi-platform GUI development. (Perl) [OS Independent]

Tango Desktop Project - Aims to create a consistent graphical user interface experience for free and Open Source software. Style guidelines, basic icon library, and list of projects using tango icons.

The Visual Component Framework - C++ framework created to provide platform GUI framework, features such as Java and Java's Swing, and Borland's Visual Component Library.