When it comes to loosening rusted bolts, some ways are more ecologically friendly than others.
Heat
Simply heating the nut will sometimes expand it before the bolt heats up, breaking the rust lock and allowing it to come off.
Chemicals
Lubricants and ordinary oils will not break the bonds of rust. Special "penetrating" or "rust-breaking" products are sold in all the renovation or automative stores. Sometimes tapping the nut with a metal object will help the penetrant to flow into the threads. Never combine heat with the penetrating oils; in certain cases it can give off toxic fumes, if not cause a fire. For details on overheating oils, click here.
Heat & Wax!
So Nigel from Halifax, Nova Scotia comes to the rescue with a technique that I would have never dreamed about, but it really works. He simply heats the problem nut and then melts paraffin wax, the stuff you use to can fruit, right into the joint. In every nut we tested it seriously loosened it up, with no toxic fumes problems.
Thanks Nigel -- that's one of the best tips of the year.