It's just occurred to me that cranked screwdrivers are available. I think they're mainly intended as a way to turn screws in hard-to-reach places, but their nature does allow more turning force on the screw. From experience, they're actually quite hard to use; keeping the business end in proper alignment with the screwhead is difficult.
I'm in whole-hearted agreement with Bugbear on the use of screwdriver bits in braces, though. For removing larger, well-stuck screws, there's nothing better, especially as the brace allows you to keep the bit in good alignment with the screw, and keep a good axial force on to keep the bit engaged in the screwhead. I've found that with bigger screws - Number 12s and above - you can use a 10" sweep brace safely enough; with smaller screws some discretion with turning force is wise!