Build your own 3D printer

RepRap is an open-source, low-cost rapid-prototyping machine. Version 1.0 is currently in development, but once it's finalised, will become a blueprint that will allow people to build their own machines themselves, with the parts costing about $400.
Many of the parts of the machine can actually be 'printed' by the machine itself, and the goal is (eventually) to make the machine completely self-replicating.
You can listen to a talk about the project here, or visit the site here.