From easiest, but least flexible to more options, but very complex:
- 3DTin - Currently my student's favorite. This browser-based modeling program has a very simple interface - just choose 3D shapes from a library add them to your sketch. Free because "when you create content on 3DTin.com, it becomes a publicly available artwork under Creative Commons Attribution-ShareAlike license (3.0) (unless explicitly specified otherwise)." All sketches can be exported in STL, OBJ, DAE file formats. [UPDATE 9.19.13 - 3DTin was acquired by Lagoa. We'll see if / how this matters.]
- TinkerCAD - It was suddenly gone, then just as suddenly back, thanks to Autodesk. Exports to STL.
- SketchUp - Exports to STL
- Autodesk software - industry standard. Free to educators and students.
Other (not-free) tools we know about but have not yet explored include:
- Shape-It-Up -a program that lets the user build shapes from hand gestures using a Kinect 3D camera. And I missed the demo at Maker Faire?!
- Solidworks - industry standard CAD program - with, I believe, a price to match?
Other resources (ideas?) to be added soon...