The early to mid 90's MTB's (non suspension) are excellent commuters (Like Andy K mentioned). Otherwise, Craigslist, Coops, Police auctions, pawn shops, LBS that sell used, garage sales and etc. are all places you can find great bikes. Look for name brand like Trek, Specialized, Giant, etc. They may have cheap components, but most of the aftermarket parts should be easy to find, replace, or repair if needed.

You mentioned you go to school. If it is a college go to the police department and ask what they do with bikes they collect that seem abandoned. I was walking around a local campus and noticed police stickers stating the bikes will be confiscated by a certain date if no one picked them up. I have my eye on a few bikes.