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/

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

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

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

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

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

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

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

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