Hi David, Thanks for the help, but I am aware of these documented features. I was hoping to learn about more "secret" ones. :-)

In particular, it would be great to discover some tools for fattening curves, in a similar way as to how Extrusion, ThickSurface, or FilledSurface can fatten a surface into a 3D object suitable for printing. Currently, I use Tube[curve,radius], and manually position spheres wherever the curves connect, but this can easily become complicated and obnoxious. Other, documented features are not so helpful. For example,

output = Tube[{{-1, -1, -1}, {0, 0, 1}, {1, 1, -1}}, 0.4];
output = Graphics3D[{CapForm["Round"], output}]
Printout3D[output, 
 FileNameJoin[{NotebookDirectory[], "rounded_tube.stl"}]]

or

output = Tube[{{-1, -1, -1}, {0, 0, 1}, {1, 1, -1}}, 0.4];
output = Graphics3D[{JoinForm["Miter"], output}]
Printout3D[output, 
 FileNameJoin[{NotebookDirectory[], "mitered_tube.stl"}]]

will produce lovely-looking output, but the caps and joins are apparently just part of the display preferences and not part of the 3D surface generated by Mathematica. If you take those STL files into a slicing program, you'll immediately see the failure: the caps are not present and part of the surface near the join is missing entirely. So they are not useful for 3D printing at all.