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.
Earth orbits and satellite coverage display in 3D on your laptop.
SaVi - simulate Starlink, Iridium, Globalstar, O3b, GPS, Galileo and other satellite constellations, modify them, or design your own. Requires Tcl/Tk and Unix libraries; 3D viewing option requires Geomview running on X Window.
surfit is an open source and free gridding and contouring software. surfit can do interpolation/approximation of points, contours and surfaces, can take fault lines and trend surfaces into account, can use inequalities and other conditions. Supports a lo
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.
The 3D Slicer is freely available, open-source software for visualization, registration, segmentation, and quantification of medical image data. Follow the link for more info: http://slicer.org