Zipper/zip slider
The zip on my jacket broke, but fortunately, I had recently finished building a 3D printer! So, like any good 3D printerer, I took the broken zip parts, measured them up, modelled them in Solidworks and printed a new one! It works great, and prints in less time than it would take me to drive into town, buy a new zip and come back!
http://www.youtube.com/watch?v=-SNgIFB3kvM
(Sorry for the Solidworks model, I'm planning on getting familiar with free 3D CAD packages so that anyone can modify the design.)