TinkerCad is a free web-based CAD application provided by Autodesk. It is an excellent tool for small 3D modeling projects which do not require high precision. For instance, it can open an STL model that was created in Mathematica, and then add annotation, combine the model with other models, and transform models.

TinkerCad does have some technical limitations, such as its ability to handle only small mesh surfaces for partitioning model components, and an occasional  technical error when it performs successive rotations of an object which are not multiples of π/2 causes skew of an object. In general; however, for many applications it is an excellent free tool.

Tutorials:

• Tutorial for text annotation of 3D models.
• Tutorial for using a mesh surface to partition an object.