OpenJSCAD.org (also known as JSCAD) provides a programmers approach to develop 3D models. In particular, this functionality is tuned towards creating precise models for 3D printing.
JSCAD provides
- ability to create and manipulate 3D models, as well as 2D models
- ability to use JavaScript programming concepts, and libraries
- ability to save 3D models as STL (and other) formats