One of the most promising uses of 3D printers before we get to that multimaterial and electronics utopia, is connecting things, interfacing things in the physical world. The Free Universal Construction Kit (note the acronym) is a brilliant example of this, but the possibilities extend much beyond toys.
One example that springs to mind is allowing any camera to take photos down microscope, by creating a holder than can hold the camera in the right place. There are many further examples in the laboratory, construction industry (e.g. plumbing), DIY, home manufacturing, and related fields, where tools and components do not interface with each other, limiting functionality.
Another area is extending the functionality of tools via 3D printed add-ons - the physical world equivalent of browser plugins.