I recently switched from a rigid mtb to a drop bar bike with fat tires (I'm running 26 by 1.5); this allows me to have what I think is the best of both worlds. I recently built the bike up from a frame I had sitting around; it's a 1993 Bridgestone XO-2.
There are plenty of road bikes out here that take a fat tire and which make a good commuter. A cross bike like the surly cross check will work well; so would a touring bike. Alternatively (and a lot cheaper) just find a vintage road bike that takes fat tires. You won't pay a lot and you'll have a great commuter.