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.
...Finally I decided to make one of my own (the world waited for that so badly!). And here it is.
Since I did not want to restrict it to Windows only I made it a GTK application, which is cross platform portable. I created the low level data gathering functions for both Windows and Linux. You are very welcome to add additional ports (e.g. BSD or whatever).