Well, if you made the mistake of putting decking down with stainless steel screws like I did, you may have run into this issue before. Either you didn't drill a decent pilot hole, or it was getting to be too hot, and you were in a rush, it doesn't matter. The bottom line is that you now have a completely stripped stainless steel screw head staring you right in the face.
After giving up with the hacksaw (because I realized that would just leave a dead screw in the wood, I decided to use an old flathead screwdriver bit. I held the bit on the head of the screw and pounded it into the screw a few times with a hammer.
After the bit was into the screw, I slowly turned it out.