BRL-CAD is a powerful cross-platform Open Source combinatorial Constructive Solid Geometry

I was listening to FLOSS podcast and the developer for bzflag also is working on the open source 3D CAD program BRL-CAD, very interesting and it is cross platform.

BRL-CAD

BRL-CAD is a powerful cross-platform Open Source combinatorial Constructive Solid Geometry (CSG) solid modeling system that includes interactive 3D solid geometry editing, high-performance ray-tracing support for rendering and geometric analysis, network-distributed framebuffer support, image and signal-processing tools, path-tracing and photon mapping support for realistic image synthesis, a system performance analysis benchmark suite, an embedded scripting interface, and libraries for robust high-performance geometric representation and analysis.