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.
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.
The Lightweight Java Game Library is a native binding for OpenGL and OpenAL written with game developers in mind. It allows the full OpenGL and OpenAL APIs to be exposed without the penalties associated with competing methods, nor the limitations of APIs such as Java3D. Tutorials and demos are available, and Windows, Linux, and Mac OS X are supported.
Red-Piranha is a search system that can actually learn what you are looking for. It can be used as a Web page, command line, or XML-WebService, so it will work with most languages, including Java, Perl, C#/.NET, and PHP. It includes learning abilities for the Desktop/Internet search functionality. All feedback from the user is stored in (editable) XML and RDF, and is used by the system to improve the quality of searches.
Netclasses is an asynchronous networking library that works with GNUstep and Mac OS X. Out of the box, it gives a nice, asynchronous, and object-oriented interface for IRC, line- based, and raw TCP/IP connections. Users can easily subclass the raw TCP/IP or line-based interfaces to create asynchronous interfaces for other sorts of connections.
Style is the ongoing C++ port of IBM's SWT for Java, itself deriving from IBM's VisualAge for SmallTalk, with two major twists. First, it is written in 100% ISO-C++ with no specific compiler dependencies, and second, it comes with a threading model design that guarantee that users won't be stuck with busy cursors forever. It currently targets OS X, Win32, and GTK.