1) People are collaborating on the Internet to build prosthetic limbs. These are produced so inexpensively that as a user grows they can easily be replaced.
2) Robotics - Hobby and education robotics are a great way to interest kids in science, engineering, and programming. Desktop 3D printers pair nicely with the recently available small computers designed for hobby use, like the Raspberry Pi and Arduino. The robots that can be built by hobbyists and schools are improving greatly.
3) Education: Telescopes and other instruments can be collaboratively designed and then produced by anyone with access to a 3D printer. So can visual reproductions of terrain, or architectural models, or biology models.
4) Combined with CT scans, 3D printing can produce components specifically tailored to an individual patient.