Shapeways now offers free, Creative Commons licensed mechanical parts models that we can use in our projects. I’m sure they’d love it if we used these CAD models to build the contraptions of our dreams on the computer and then printed them through Shapeways, but even if we don’t, the virtual models are ours to keep.
I haven’t built any 3D printed mechanical models yet, but I’ve been impressed by the results of other designers. Check out this functioning roll-back racer. This is science fiction, right?! Unlike the excellent Thingiverse, this is not a community-driven database, but Shapeways says they plan to allow user-contributions in the future.
The files are in the STL format, which most 3D modeling packages I’ve dealt with can import.