If it's really accessible and there is still a fair amount protruding from the hole, try as mentioned cutting a slot in it and using a screwdriver to turn it out.
Failing that, there are a few options to extract it. You can use left-hand drill bits or an EZ out. Most commons types are shown here:
http://www.toolprice.com/category/screwextractors/
I recommend going straight to the EZ out if there isn't enough material protruding.