Craftsman Impact Driver - Model 47641 at Sears.com
This thing will be your go to tool for stuck fasteners of all types. It is alot better then trying to drag a compressor and air tools out to the boneyard.
If you have a stuck phillips or flat that you can't break loose with hand power alone, leave it and grab this, a couple sprays of pb blaster and a couple whacks with a hammer and this thing will turn that screw right out. Also works with 3/8 drive sockets. I have only met maybe two fasteners that this thing couldn't remove and those ended up needing a bit of torching to remove them.