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aviation and aerospace applications, sports equipment, racing industry.

I also see possibilities in urban design, using less material for the same structural integrity.

Making tangible prototypes of designs that wouldnt be feasible to create without 3D printing

I do not own a 3D printer yet

3D printing in space.
Made in Space recently launched their first printer on the ISS via SpaceX's Dragon, which itself will have 3D printed engines.
Other rocket engine manufacturers are testing...

I'd use it to make photo/card holders. It's something that you wouldn't go out of your way to buy, plus it might be cooler to design your own.
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First thing that came to mind was conductive materials.

Going on a slight tangent, but it would be cool to see filament/software combinations to create gradients on a print - whether the color...

I suppose this is Ebay/Amazon for 3D printers. Very interesting idea on selling artifact prints - I wonder if 3D printed replicas will adversely affect the antique store industry

- What is the most interesting thing you have seen done with a 3D printer or 3D printing technology?
Printed internal gears/ complicated internal parts that would not be possible without additive...