DiaCanvas is a generalized version of the drawing canvas used by DIA. It extends some features used by DIA and adds new ones, while preserving as many of the original features as possible. This project is no longer actively maintained. It is suggested that you try DiaCanvas2 instead.
ClassyTk is an extension of Tk based on the ClassyTcl Object system. It adds a lot of improvements to Tk, ranging from drag and drop support (between Tk apps) and a configuration system to a large set of new widgets and commands, written using the object system. It also contains a GUI Builder that can be used to graphically create interfaces. It is invoked via the cbuild command in the bin directory. There are also some sample (but useable) applications created with ClassyTk: a very useable editor, a calculator, a drawing program, and more.
Papyrus is a 2D C++ scenegraph (canvas) library that utilizes the cairo library to perform drawing operations. Similar canvas libraries are the Gnome Canvas, Diacanvas, and the QT Canvas. Numerous canvas primitives are available, including arcs, rectangles, polylines, and line markers. An optional companion library, Papyrusmm, provides a set of Gtkmm widgets which includes viewports (a canvas display widget similar to Gtk::DrawingArea) and other UI widgets related to drawing interfaces.
Moss is a collection of Java classes which includes additional custom Swing components (Tri state check boxes, complete password entry dialogs, a JFrame/JDialog framework to assist in creating large GUIs), file and image modification classes (EXIF/IPTC reading and editing, image roatation, file copying), miscellaneous data structures (such as a HeapSet implemenation), and much more.