I mentioned in Visual Anthropology class that a cheap lavalier (clip-on) mike is ideal for doing both audio and video interviews. The cheapest that I found that should be sufficient for the purposes of the class is the Sony ECM-T6 for $12.95:
Sony ECM-T6 - Omni-Directional Lavalier Condenser Microphone - $12.95 (B&H)
The next level up is the Sony ECM-C115 for $39.95
Sony ECM-C115 Omnidirectional Lavalier Microphone - $39.95 (B&H)
Both of these have their own on-board power-supply (CR-2025 batteries) so they work with camcorders, audio recorders, and laptops that provide power (aka Plug In Power) as well as ones that don't. If you're going to use your laptop for recording, chances are it doesn't have Plug-in-Power, so you need a mike that is self-powered.