OpenPCR is a fully functional yet affordable ($599us) PCR (polymerase chain reaction) device, used for replicating DNA for the purposes of sequencing or barcoding (species determination). The whole kit - from the software to the hardware itself - is open source, so you can download the code, CAD, and Eagle files and build your own if you don't want to buy a kit.
RepRap is an open-source (hardware, software, and documentation) desktop fabrication system which lets you construct arbitrary objects (or parts for assembling more complex systems). It's unique in that you can construct one yourself for about $400us, and it is fully capable of making duplicates of itself.
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