I haven't looked at all these free 3D tools but some may be of interest to those heading toward 3D printing. Not sure they all output files in the appropriate format(s). Nonetheless, some of these links might be useful to some inventors.
After last year's CES, I noted that makers of 3D printers and 3D printing service provders had a small but noteable presence. This year their booths seemed larger and at least when I came by, the crowds around the booth seemed larger and more engaged than last year.
If you haven't looked and your invention involves "things," you might want to have a look at the growing market for 3D printers and related services. Here is a short list of those I looked at @ CES. Look here for a basic view of 3D printing (stereolithography).
For those in Silicon Valley, the SVForum has established a 3D Printing SIG. Their next meeting is January 29th.