CockTail is a Java RAD & WORA Tool used to develop GUI & Swing based user interface for applications

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

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

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

An engine that reads XML files describing graphical user interfaces layout.

Open source GUI for less powerful or old computers.

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

NeXT Step alike window manager for the X Window system on GNU/Linux.

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

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

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