[chris vickery]

There is also a tool which is hand impact driver of sorts, typically used on things like auto door hinge screws etc.
Look up Lisle 29200 on the internet to see what I'm talking about. Other companies such as Proto, SnapOn etc make them as well.
If you opt to purchase one, buy a good one.
It works like this;
You have a handle which has a striking surface at one end and a hex driver on the other. There are some mechanics inside which allows it to be used for driving in or loosening out screws by twisting the shaft one way or the other to change drive direction.
They usually come in a kit with various types of bits including the parallel ground type slotted bits.
You place the appropriate bit in the driver, place it into the screw and strike the end of the handle with a large hammer. I am sure if the bolt were heated it would help. I have used it in the past and it worked great.
I have often found as well that it benefits to try tightening rusted bolts a bit before trying to loosen them.