I recently picked up and put on some Serfas Vermin tires from my lbs. They ride very smooth and I am very pleased with them. They have a smooth rolling light tread, with some lug action on the shoulders in case the tire should sink down a little. They also have an embedded protective belt in case you should run over something too sharp. I've ridden them a few times, unfortunately its been too cold for me to put significant miles on them, but that will change shortly. They are a leap and then some ahead of my knobby Roll-x I had on there before.

EDIT: I was also looking at the Maxxis Overdrive's, but the tires above my lbs had in stock and he recommended them very well. Plus, having the side lugs vs the Overdrives, at least if I get into something soft like the washouts on the Hennepin Canal Bike Trail here in Illinois, I hopefully won't be *AS* much of a turtle. The Serfas is a 2" tire which looks and feels pretty decent.