This neat article came across my Twitter feed this morning – a DIY spectrometer for $35! You can view the plans here, and the creators have started a Kickstarter project to build a public database of spectra for comparison with unknown materials. The creators have even written open-source software to collect and analyze spectra from their spectrometers. For those in the know, the spectrometer has a wavelength range from 400-900 nm and a resolution as good as 3 nm. You can either buy the parts yourself or buy a kit from Breadpig.
By the way, the site hosting the plans, Public Lab, is an open community that investigates environmental issues with DIY tools. The site is quite a bit of fun to poke around, and I will be re-posting some of their more interesting educational ideas.