Andreas Herz has added diagramming features (connectors, ports, tool selection) on top of Walter Zorn's famous DHTML graphics library to create Open jACOB Draw2D. There are several interactive demos that let you create simple diagrams. There is also javadoc style documentation (though it mostly documents the class hierarchy).
DIVs. Because the library doesn't keep track of the
DIVs associated with each primitive, there's no way to undo a shape (short of clearing the entire drawpad). wz_jsgraphics can also be slow (anything but rectangles and horizontal / vertical lines requiring large number of
DIVs). However, these shortcomings aren't fatal for a diagramming application.
Open jACOB is supposed to be a bigger framework, including a server-side part, though at the moment there isn't much to see besides Draw2D. Overall, this is a promising start; hopefully this project will stay active and yield (further) useful results.