My commute includes 4-6 miles of dirt and gravel roads/trails (depending on which route I take). Knobbies aside, I regularly ride Vittoria Randonneur Hyper (700x32, 35) and Conti Contact Extralights (700x42). The former are slicks, and work fine as long as you drop the PSI somewhat. The latter have a mild tread, and work quite well on a wide range of surfaces. Note that the Hypers run true to size, whereas the Contis run narrow (by 3-5mm).