I've been using one of those handheld multi tools with the circlar disk saw to saw the bolt off. However this only works where there is a nut and bolt and you don't need to get the whole bolt out of something solid.
If it's really bad then grind it off, heat it up or weld a bar to it. Welding seems to work well as the heat helps break the seal/rust etc. It worked on my seat belt bolts which were corroded in the chassis.
