GUI

FOX GUI Toolkit
An open source cross-platform C++ based Toolkit for GUI development.
http://www.fox-toolkit.org/

GUIS
Open source widget server. (Python, Ruby, GTK2) [OS Independent]
http://starynkevitch.net/Basile/guisdoc.html

InputGUIDO
A GUI being developed for older DOS systems that are incapable of running Windows.
http://www.trhonline.com/input/

Java Gui Builder
An engine that reads XML files describing graphical user interfaces layout.
http://jgb.sourceforge.net/

KronoJunior
Open source GUI for less powerful or old computers.
http://kronojunior.sourceforge.net/

ObjectCentral - V
A free, multiple platform C++ graphical user interface framework.
http://www.objectcentral.com/

The Official FVWM Homepage
The standard window manager for linux, stable but not as pretty as Window Maker or Enlightenment.
http://fvwm.org/

Prima
Toolkit for multi-platform GUI development. (Perl) [OS Independent]
http://www.prima.eu.org/

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.
http://tango.freedesktop.org/

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.
http://vcf-online.org/