A PhysicsBuzz article show us how to build a spectrometer with simple stuff that we have at home!
"A CD has the very cool property of behaving like a reflective diffraction grating. An ordinary diffraction grating is a grid of tiny, evenly spaced opaque lines on an otherwise transparent material. Light can pass through the material but it has to bend around the lines, which are about the size of a wavelength of visible light." in PhysicsBuzz