Official Window Maker Website

NeXT Step alike window manager for the X Window system on GNU/Linux.
http://windowmaker.org

The Official FVWM Homepage

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

Welcome to 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.sourceforge.net

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/

ObjectCentral - V

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

FOX GUI Toolkit

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

KronoJunior

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

Prima

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

GUIS

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

CockTail - The Java RAD & WORA Tool

CockTail is a Java RAD & WORA Tool used to develop GUI & Swing based user interface for applications
http://www.applese.com/products/cocktail/cocktail.html