You’ve probably already heard of RepRap, Fab@Home and CandyFab. All three are ‘Open’ projects (all the building plans are available for free) and they aim to design and build cheap 3D printers that will be able to replicate themselves (RepRap), or to print out delicious candy (CandyFab).
They are all very cool, but some people are going low-tech and they’re using what they already have in their own home: LEGO! Check out these two tutorials on Instructables.com on how to turn your old pile of LEGO into a functional 3D printer!
You’ll need to write your own control software for these though (or maybe snag it from RepRap or CandyFab?).