I screwed some steel brackets into some slabs around my pond a few years ago to attach a fence. The fence is down now but I can't get the screws out to remove the brackets, I've tried electric drill/screwdriver but no luck...
If the screws have been in a few years CK1, If the head is visible on the screw, you can use mole grips and clamp it out, other than that you will have to Re-drill the screw out, drop some oil on the screw and leave for a day or so and let the oil penetrate, you may find the screw snapping within.
If you cant unscrew the screw, get an electric drill and drill the head off the screw, a bit tricky to get started but works every time, as long as you don't mid leaving the remainder of the screw behind.
Chances are that if you've used an electric drill and not an impact driver then the head (especially if it's a Phillips) will be worn. I don't think you need anymore advice. Everyone has supplied sufficient methods. The drilling method is the easiest in my opinion although as TWR has said if you can get hold of the screw a pair of mole cribs will work.
society: I love screwing. There is nothing like getting out your tool and doing some screwing. Unfortunately I don't have anyone to help me use my tool so I don't screw anymore. Rather lonely.
TB: As much as I like your answer and given you are in the trade as I am I doubt ck has access to such a tool. Drilling the screws out would seem the most logical method.
I've removed a countersunk bolt before by drilling a 5.5mm hold through the bold head and tapping it (threading) with a 6mm anticlockwise tap. After dyeing a length of bar with an anticlockwise thread I managed to use that to remove the bolt. It all sounds simple but it requires the tools for the job :)