GGredit is library that implements base functionality for a generic graphic editor. DDraw is a diagram editor designed for Gnome that use GGredit. DDraw is inspired to DIA, it can be used to draw many different kinds of diagrams.
K3Studio is the universal workbench for 2d/3d modeling, visualization and simulation.
The main topic is simulation and visualization of automata networks, but it can be used for diagram drawing, flowcharting, presentation, as a CAD, GIS,...
ydiakun runs on a X Window system with Gtk+ library and draws a train operation diagram. It reads and writes WinDIA format data file. WinDIA is commonly used among Japanese railway enthusiasts.