freeglut is intended to be a 100% plug-compatible clone, and is released under the Xfree license. The GLUT (OpenGL Utility Toolkit) is a widely used library for doing window control, keyboard, and mouse input, and other basic functions for OpenGL programs. However, GLUT is not OpenSourced - although it's source code is available under a somewhat ill-defined set of restrictions. Hence, 'freeglut'.
Java Supplements is a free library of utility classes which extends and enhances the functionality of the classes provided by the normal Java API. It provides robust and reliable code for doing many of the tasks which Java developers commonly find themselves writing code for again and again, such as catching InterruptedExceptions on Thread.sleep() calls or reading files into a byte array.
guitest is a collection of helpers for unit-testing GUI applications written in Python. Support for pyGTK is most mature, and support for other toolkits is in progress. The library allows testing of modal dialogs and recursive main loop invocations while preventing the windows from being shown during test runs. Arbitrary classes and methods can be overridden, or call loggers attached to them.
libview is VMware's Incredibly Exciting Widgets, a collection of GTK+ widgets used within VMware products. These widgets are the same ones used in VMware Workstation 5.0 and higher, and were considered useful enough to open source so that others could use them in their own projects. These widgets are actively developed and maintained.
gbdfed is a GTK+-based BDF font editor. Multiple fonts can be loaded from the command line. Multiple fonts can be open at the same time. Cutting and pasting glyphs between fonts, font name and property editing, and built-in online help. It imports PK/GF, HBF, PSF, CP, Linux FNT, VFONT, OpenType/TrueType, and some FON/FNT fonts. It can export PSF2 fonts.