[Animats]

Not sure what the author means by "freeform". He seems to use that term to refer to mesh editors, which do surfaces for graphics, vs. solid geometry editors, which understand volumes. Mesh editors are more for 3D graphics.

Learning to use a constructive solid geometry system used to be hard, but Autodesk came out with low-end tools based on their their Inventor product line which make it easier. There's 123D, 123Make, 123Sculpt, Meshmixer, etc. The heavy CSG machinery of Autodesk Inventor is inside, but it's not dumped on the user all at once.

[delhanty]

They also have Inventor Fusion, which is gratis (or at least used to be) on the Mac App Store - not sure what the Windows version costs. That and OnShape are probably the most capable $0 options for parametric solid CAD.