Folks are also fond of soaked (submersed 24 hours) phone books, generally held together by duct tape.
Simple gallon milk jugs filled with water back to back to back, etc, as appropriate will also work most times.
Believe it or not, gummy bears/vitamins will stop a bullet surprisingly well. My uncle once had a bottle of gummy vitamins that was accidentally left in the car. The solidified block was no more than 1/2"x2"x4" and it EASILY stopped multiple .204Ruger bullets fired at over 4,000fps from a distance of just a few yards. It's almost incomprehensible to me and I saw it with my own eyes. Finding the bottle afterwords was another challenge altogether.
If you have one, a 55 gallon drum filled with water and shot from above will stop MOST bullets. Just don't be too in love with the drum and consider the possible directions that the bullet could deflect through the sides.
