Put an ssd in a computer for a friend and needed rails to put the 2.5" ssd in a 3.5" bay… Mp3dp made short work of the problem.
Nice! Now cases usually include holes in the bottom of the tray for 2.5in drives and/or random ssd mounting positions in the case.
But, the lack of such features is just an excuse for us to play with our machines 
I recently built a lightweight curtain for my bedroom window and used parts that I designed and 3d printed to hang the rod up. I could have bought something that worked for $12, or spend 5 min designing my own and <$1 in plastic making my own.
My favorite use for 3d printing now is making drill templates for (mostly) accurately positioning holes in parts repeatedly.