Yes, it is apparently possible to build a working 3D printer out of Lego blocks! Don't believe us? Check out the link below to Instructables, where Gene Hacker provides a step-by-step process for creating the device.
Interestingly, he used a Polar coordinate system rather than the traditional XYZ scheme, because "I didn't have enough Legos to build a Cartesian printer". As they say, necessity is the mother of invention.