Qt is a comprehensive, object-oriented development framework that enables development of high-performance, cross-platform rich-client and server-side applications. When you implement a program with Qt, you can run it on the X Window System (Unix/X11), Apple Mac OS X, and Microsoft Windows NT/9x/2000/XP by simply compiling the source code for the platform you want. Qt is the basis for the KDE desktop environment, and is also used in numerous commercial applications such as Google Earth, Skype for Linux, and Adobe Photoshop Elements.
GTK, which stands for the Gimp ToolKit, is a library for creating graphical user interfaces. It is designed to be small and efficient, but still flexible enough to allow the programmer freedom in the interfaces created. GTK provides some unique features over standard widget libraries.
GNOME is the GNU Network Object Model Environment. This project is building a complete, user-friendly desktop based entirely on free software. This desktop consists of small utilities and larger applications that share a consistent look and feel. It uses the GTK as the GUI toolkit for all GNOME-compliant applications.
Luxor is an open-source XML User Interface Language (XUL) toolkit in Java that lets you build UIs using XML and also includes an ultra light-weight, multi-threaded Web server, a portal engine, and a template engine. It is also Web Start-ready, as everything fits in a jar and requires no loose files.
PicoGUI aims to be a complete GUI environment for handheld computers and other embedded systems. It uses a client/server model, like the X window system, but while an X server is given raw drawing commands, the PicoGUI server integrates a widget set, making PicoGUI clients small and efficient. PicoGUI also has the goal of allowing client/server connections over a variety of mechanisms. It is most commonly used with Linux, but is designed to be portable to any OS. PicoGUI has a variety of video and input drivers which allow it to interface with the Linux framebuffer device, SDL, the X window system, and several other devices.
SwiXAT is a Swing-based authoring tool for the quick and easy development of GUI Java applications. It implements a true MVC framework and uses XML to define the view (SwiXML is used as the XUL engine), BeanShell as a scripting language for the controller, and JXPath as the binding mechanism between the view and model. It provides a complete environment in which it is possible to almost write an entire Java Swing application without writing Java compiled code. The use of XPath makes it very simple to traverse the object tree of the application's business model.
The goal of the General Applet Interface Library is to give programmers a simple yet powerful applet interface. This library supports wmapplet/dockapps, GNOME 2 panel applets, and ROX panel applets. The applet programmer doesn't have to care about where the applet will be used, since the library handles that. Over 50% of the code in a dockapp and a GNOME 2 Panel applet does only one thing, setting up the applet window. With GAI, this can be reduced to just a few lines of code.