Only works if part is off the motor, but I remove frozen studs, bolts and stuff like that from aluminum by throwing it in my nitric tank. Eats the steel in a few hours and doesn't attack the aluminum. So if you got an old aluminum part with a stuck bolt find your local plater, he'll get it out.
I've had good luck with a little heat, either from a heat gun or small torch. I got all the plugs out of a set of aluminum heads that had been sitting off the car for 30+ years by soaking them with some penetrant for a couple hours and using a heat gun on them.
I've also seen a cool trick where the part ( a sump plug) was warmed with a torch and then wax from a crayon put on it, as it cooled I guess the wax must have got into the thread, or something, but the plug came out easy.
Wax works killer for getting stuck stuff unstuck. I didn't believe it untill I tried it. Now I'm one of the converted