A tool for Constructive Solid Geometry like OpenSCAD but written in Sage http://www.sagemath.org/
There is already quite some functionality in very little code.
You can (via the Jmole 3D viewer in sage) preview & turn the 3d output before saving to *.stl file. With Sage you have a fully blown computer algebra system CAS at hand. Transfinite unbounded objects can be used for construction.
Sages implicit_plot3d is not made for this purpouse so I think this is more useful for experimentation with system design than useful construction work. But I might be wrong.
In this context I like to mention
ImplicitCAD (written by Christopher Olah)
which Is ment to deliver good performance.
I started to code miniSageCAD because while trying sage it became patently obvious to me how easy It'd be to get a minimal useful CSG system.
I wanted to preserve symbolic derivability for all csg expressions (to have symbolic access to the surface normals). I found that only lambdas are symbolically derivable but those lambda expressions can not have line breaks. This makes it necessary to break more complex objects down into subfunctions. A good thing for documentation (names) but bad in that it clutteres the global namespace. Also obstucted by this is the idea to e.g. transport the positions of the edges of a cube up the csg graph accessible in e linearly-transformed way.
new in v0.04:
arbitrary arity union and intersection
prism rod, prism
inshell, onshell, outshell
gyroid (left & right)
Either try it via the public worksheet (note: they are currently disabled 2012-01-04)
http://www.sagenb.org/home/pub/5036/ (Gone down somewhen before 2016-06-12)
Or download it and use it within your own sage system.
If you make somthing with it please upload your results.