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

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.