GUI

FOX GUI Toolkit :

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

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

- A GUI being developed for older DOS systems that are incapable of running Windows.
Java Gui Builder :

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

- Open source GUI for less powerful or old computers.
ObjectCentral - V :

- A free, multiple platform C++ graphical user interface framework.
The Official FVWM Homepage :

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

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