I ride my mountain bike almost exclusively on the road. For one thing, there's virtually no off-road riding to be done around here without trespassing, and it's the only bike I have right now so I have to ride it.
I used to have some cheap slick tires on it, but I recently got some Kevlar bead Continentals (can't remember the model name) and they have really short knobs and can take fairly high air pressure, so they really don't create much rolling resistance (for an MTB tire). They're actually quite a bit lighter than my slicks were, and I can go through rocks/dirt/mud if the need arises. lol