It was only a matter of time: some folks at Michigan Tech university have released an open source design for a METAL 3D printer that uses a MIG wirewelder as the extruder.
Now before you get out you checkbook, take a good look at the sample part image in the above article; yeah the resoultion is fairly low and you won't be making crisply finished stuff just yet in comparison to the other methods like laser sintered metal. Cost-wise it almost gets you cast aluminum-like level of details in carbon steel or stainless steel. However my feeling is people will continue to improve this rapidly so in another year or so it will have much better resolution for details. This does make a good starting point though for further machining.
Full details/ plans here: http://www.appropedia.org/Open-sourc...#Printed_Parts