I think RepRap et al could have a huge impact in poorer countries. Spare parts for out of production goods can be impossible to find, and replacing an old (regular paper) printer or something like that is a big cost. Imagine a small factory in every village. :-)
Fablab is also a very interesting project. They provide free access to 3d printers, scanners and other equipment, provided you share your design. GPL IRL!
Easy maintainability is a general policy in third world countries already though, eg India makes 1950s design british motorbikes and cars (Norton, Morris Oxford) because they can have replacement parts made by low tech means.
However 3D printers potentially do make more kinds of materials buildable, especially when combined with printable electronics.
Fablab is also a very interesting project. They provide free access to 3d printers, scanners and other equipment, provided you share your design. GPL IRL!