I own four commuter bikes. Well, I own three. My wife owns the other one.
Any bike that's usable for commuting is a commuter bike.
Trek 1200 with rack, panniers, fenders and lights.
Diamondback Outlook with slicks, rack, fenders and lights (also added clipless pedals and better rear cassette for higher speeds)
Diamondback Sorrento mountain bike for winter commuting and hitting the trails.
My wife has a Townie 3 Speed.