This wonderful video by Kyle Ronan shows a home-built RepRap printer in action. RepRap is an open source project that intends on providing the design for an easy-to-build 3D printer that is capable of reproducing all the parts that it's made from. And, it's actually already done this! Still needs some assembly, however.
Back to the video. Who knew these things were so noisy? Now we understand why the commercial printers always come with rather large and complete exterior cases!