Most interesting thing I've seen done is making replacement human organs. Imagine not having to wait for a donor kidney or heart, and not having to take anti-rejection medication for the rest of your life after a transplant.

My dream 3D printer would function about the same as the replicators on the USS Enterprise. Tea, Earl Grey. Hot. Basically convert energy directly into matter - any matter in any configuration, atom by atom. Of course it would be just as fast - basically make anything in a few seconds.