From Wikipedia: "A contact microphone is a form of microphone designed not for picking up airborne sound waves, but rather to act as a transducer which picks up vibrations through solid materials and converts them into audible sound. Often used as an acoustic leakage probe, and known by the more technical term as a 'piezoelectric vibration transducer'. It also has been widely used as an instrument by noise music artists."
Quite possibly one of the most used items in a noise artist's arsenal a contact mic is not only easy to make but also easy to use. The mic can be attached to just about anything from blenders to pieces of scrap metal to Shaker Boxes and beyond. Because they work on vibrations most stationary items are shaken or beaten or kicked around to generate the sounds that can be further processed.
You can easily purchase a contact mic (or drum trigger) but that just doesn't live up to the DIY aesthetic. Construction is achieved simply by taking a piezo buzzer (which can be purchased from RadioShack or procured from any number of sources including musical greeting cards) and soldering either a jack or cord to it. Plug it in and go. Different sized piezo buzzers yield different results.