The Pure Data (short Pd) project on SourceForge unifies the extensions (or externals) written for the Pure Data Graphical Computer Music System written by Miller Puckette with contributions from many others.
BRL-CAD is a powerful cross-platform constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing for rendering & geometric analyses, network distributed framebuffer support, image & signal-processing tools.
tkBacktracer is a cross-platform interactive visualization tool to explore the tool motions in RS-274D "G-code" CNC files used in computer controlled milling and manufacturing.
Open Source and Portable object oriented software for 3D digital image segmentation and visualization. Pipes and Filters Architechture.
Implemented in C++, and uses FLTK, VTK, ITK, TCL (plugins), and pipeline filters implemented in XML. DICOM support.
Powerful platform for creating networked multiplayer 3D games. The system includes specifications for a client engine and 2 protocols. Demo apps: mud/moo proxy, tetris. Currently uses OpenGL, VRML, java, tcl. Client/server architecture for security.