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.
Root3D is very convenient and beautiful cross-platform 3D filemanager. It uses Java platform and OpenGL to work. It includes such parts as filemanager itself, multimedia player, text editor, photo viewer, and other office plug-ins.