Of course, I missed the most practical use of 3D printing, and the one that prompted NASA to send one to space: Replacement parts for things that break.

How much do you pay for the replacement hose clamp in a dishwasher, waiting days for it to ship? The kind of thing takes minutes to model, maybe an hour to print, and under a dollar worth of materials. Also switches, cable/hose holders, and panel holders in your car are all perfectly acceptable replacement parts to print, which in many cases cost upwards of $20 for a tiny part and sometimes aren't available at all.