Like Andy K said, a 1980s or early '90s steel frame mountain bike makes a great commuting bike if you stay away from suspensions of any kind. Most of those old MTBs had eyelets for fenders and racks, which is really helpful. In addition to Craigslist, try looking in thrift shops and sports consignment stores. If you don't know much about bikes, find a knowledgeable friend to help you pick something out. One nice thing about these bikes is that you can get them cheap, and replace stuff as you deem it necessary. This is much easier on the wallet than plunking down $300 or more for a bike all at once.