Trek 520
I think a touring bike makes a great commuter. Comfortable, rugged, and you can hang a lot of stuff on them (racks, lights, fenders, etc.). I got a used Trek 520 for commuting, and like it more than the racing-style bike I had been using. Bigger tires make me feel safer on bad city pavement, and I can watch traffic better (instead of looking for holes in the pavement, I'm looking at cars). The Trek is also doubling as my winter/rain bike -- having this bike fitted out w/fenders means that on wet weekend days I'll still head out for a ride instead of sitting around hoping for clearer weather.
I use SPD pedals 'cause I don't like riding without them. I keep an extra pair of shoes at work.