XWidglets is a complete rich Java Swing tool for designing, creating, and using XML-based GUIs. It provides a lightweight XML client, clear separation between view, data, and process, MVC I and II support, event management, a consistent look and feel across platforms, a rich set of GUI components, and n-tier architecture integration.
Peggy helps you to create GTK applications that run out-of-the-egg. It offers functions to load resources (like .glade files, images, and locales) from a zipped or unzipped egg. It is a thin layer on top of setuptools and PyGtk. It doesn't do anything very fancy, but it can save you from reading some documentation.
Wimpiggy is a library for writing EWMH-compliant, compositing window managers using Python and GTK+. The goal is to make writing a window manager as easy as writing a PyGTK application. This library can be used to build a trivial, working window manager in only about 40 lines of code.
ViewKlass is a C++ framework for developing applications that use the LessTif or OSF/Motif(tm) user interface toolkits. In particular it provides classes that support the creation of reusable components. It also provides classes that encapsulate some of the more difficult and tedious aspects of Motif development. These include classes to handle the creation of windows, dialogs, and menus. ViewKlass is an open source implementation of SGI's ViewKit(tm), and should eventually become source code compatible with a large subset of of that API.
REALbasic is a cross-platform software development environment for developers who want to create and deliver native applications for Mac OS, Windows, and Linux. For example, you can create an application for Mac OS and Linux from a Windows machine. It is intended for people with all levels of programming experience. It is object-oriented, powerful, and supports advanced programming concepts like inheritance, polymorphism, interfaces, delegates, introspection, etc.
Musca is a simple window manager for X allowing both tiling and stacking modes. It is similar to ratpoison but more friendly to the mouse and using a simpler keyboard navigation. There are no built in status bars, panels, or window decorations save for thin window borders to indicate focus. Window navigation can be mouse click to focus or entirely keyboard driven. Window tiling is manual but simple, and there are no restrictions on how you divide up the screen. It uses dwm's dmenu utility for launching apps and running various built in commands not mapped to hot keys.