Vice-grip makes a specialty wrench for stubborn bolts. It's for hex-head bolts and is a life-saver when some one has rounded of the bolt head. I would consider this a must have for the tool kit.
the jaws of the vice grip pliers has a "V-shaped" upper jaw and an inverted "V" on the lower jaw. These clamp down on three sides of the hex head and make short work of tough bolts!