You can record high-quality video on just about anything with a screen these days, but the built-in microphones on these devices aren't always up to par. If you want to make your own, Instructables user Snaffle J Bean shows off a simple method that only requires a couple parts and $US15.
The microphone uses a bunch of standard electronic parts you can piece together. A single RCA to male 3.5mm cable, two male RCA to female 3.5mm adaptors and two cheap 3.5mm microphones. Snap them together in the right order, plug it into your camera's microphone jack and you're done. The end result is an incredibly cheap way to add stereo recording and a quality boost to your video camera. Check out Instructables for a full product list and assembly instructions.
Make a stereo microphone for $US15 [Instructables]