• http://www.kickstarter.com, by Jeffrey Yoo Warren
A spectrometer may not sound like what you wanted for your birthday, but it's a ubiquitous tool for scientists to identify unknown materials, like oil spill residue or coal tar in urban waterways. But they cost thousands of dollars and are hard to use -- so we've designed our own.
This open hardware kit costs only $35, but has a range of more than 400-900 nanometers, and a resolution of as high as 3 nm. A spectrometer is essentially a tool to measure the colors absorbed by a material. You can construct this one yourself from a piece of a DVD-R, black paper, a conduit box, and an HD USB webcam.