Design it online then print it.
Dolf Veenvliet, also known as macouno, created two making apps that lets you design a ring or spaceship online then immediately upload it to be 3D printed. The ring app uses a series of sliders to change various features, and the spaceship app randomly generates a ship based on your name.
Both of these apps show how digital fabrication can be used to offer custom, or at least customized, products in mass quantities. Everyone can make what they buy specific to themselves.