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.
libDockApp is a simple library for easily writing Window Maker dock applications, also known as dockapps (those that only show up in the dock). It is very limited and can only be used for dockapps that open a single appicon for each process in only a single display, but this seems to be enough for most, if not all, dockapps.
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.
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.
libAfterImage is an image import, storage, manipulation, and output library for X. It features support for antialiased, TrueTypei, and X text, a 128-bit internal graphics engine, in-memory RLE image compression, high quality image scaling/flipping/blending, multipoint linear gradients, superior quality image output on X drawables, and much more.