I just bought some supplies from Radio Shack (and I’m glad such a place still exists, just two blocks from where we live) to build a contact microphone. A contact microphone is made from piezo disk, which translates vibration (or the compression of the piezo element) into sound. These disks, which are just flat, round pieces of metal with a ceramic patch in the center, are typically used as buzzers or as the speakers in those annoying greeting cards that play a song when you open them. Attach a mic cable and jack to them and you have a microphone.
The sound you get from one of these is pretty unique, since it must actually be in contact with an object to pick up sound. It relies on the sound waves traveling through solid objects, rather than through the air, as with a normal microphone. If you tape it to a guitar body and use it as a pickup, you get a slightly muffled yet intimate guitar sound, complete with every little accidental tap on the sound board or scrape of guitar against jeans. It is very similar to the sound you would get if you put your ear directly to the guitar itself. You can tape it to your neck to pick up your voice, a table, or whatever. I’ll try and post some recordings soon.
I built one of these a few years ago when I was living in Seattle at a “Furious Contact Mic Workship.” It was an almost life changing event, and I had a lot of fun with that microphone until it broke (piezo discs are very delicate). So I’m finally building a new pair (two, so I can do stereo recordings). If you want to know how to make make one, here’s the instructions.