From Wikipedia pages:
Rapid prototyping, is the automatic construction of physical objects using solid freeform fabrication. The first techniques for rapid prototyping became available in the 1980s and were used to produce models and prototype parts.
Solid freeform fabrication (SFF) is a technique for manufacturing solid objects by the sequential delivery of energy and/or material to specified points in space to produce that solid. SFF is sometimes referred to as rapid prototyping, rapid manufacturing, layered manufacturing and additive fabrication.
There are quite a few of these on the market, currently. Some examples are:
Z Corporation, DESIGNmate Mx and Cx, RapidPRO, Stratasys, Inc., 3D Systems, Inc..
These types of machines, once advanced enough and ubiquitous will turn the manufacturing and retail industries upside down. Of course, that realization is a long way off.
Additionally, there are a couple of open source movements to collaboratively design variants of these machines that people can build at home. The holy grail is to build a fabber that can make all the parts for itself. One of the visions surrounding these open source communities is to use fabbers to build more fabbers and then distribute them in a sort of viral grassroots manner to developing countries. Once the machines and the materials they can work with are robust enough, people will be able to make their own widgets quickly and cheaply. As the machines improve, the range of possibilities for what sorts of widgets that can be made will increase. People will be able to download a small computer file that describes how to make all manner of useful products, fittings, tools, fasteners, and the like.
Article about Fab@Home: Desktop fabricator may kick-start home revolution
Article about RepRap: The machine that can copy anything