I built my own 3D printer based on open hardware RepRap printer Rebelix X2, but improved by my own design customizations. Printer consists of plastic parts made by similar 3D printer (I used 3D printers at Czech Technical University), standard aluminium profiles, stainless steel rods and bunch of nuts and screws.
Rebelix is free desktop 3D printer capable of printing physical objects from plastics. It belongs to family of RepRap printers, which are composed of several 3D printable parts, so they are partly capable of self replicating. That means, if you have a RepRap, you can print a lot of usefull things, including parts for building another RepRap.
RepRap is open-source comunity project, everybody can build own RepRap, improve it and give these improvments back to comunity.