Its been some time but I spent about 4 years without a car. I commuted year round both in Madison, WI and SLC, UT. My favorite bike for doing so was my cyclecross. Its a Cannondale XS800 with the HeadShok.(no longer made) The cycle cross gave you a durable enough bike for anything in town and yet yet fast like a road bike. I also did the 100psi skinny slicks on a MTB. Works just as good IMO. For the winter I had a old (60's) Schwinn cruiser bike with full fenders that I did some modern upgrades in order to cut weight and increase braking power.
Anyway I'd just pick up something that has gears and slick tires. Ride the crap out of it and call it good. Spend some money on proper lighting if your going to be commuting in the dark. As well as a water proof bag for clothing or other stuff.
Speski OffRoad LTD, Cargo Solutions for Toyota 4x4's http://www.SpeskiOffRoad.com
"80 series, with a fridge"
1976 FJ40, Project with little progress.
2004 Starcraft 10RT