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.
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.