A hammer is a simple tool that can also be used as a weapon in an emergency. Most hammers are hard and sturdy enough to be able to penetrate the human skull. One handed hammers, however, require the survivor to be perilously close to the zombie, and require several swings from even the strongest wielders before the brain is destroyed. Sledgehammers are far more effective weapons due to their greater striking force, but they are also incredibly heavy, thus limiting movement and swinging speed.
One should not underestimate the value of good tools in a Zombie Apocalypse however, especially in building or maintaining a base. As a weapon, it is a good bludened tool to use in case if you want to kill the zombie from behind with the hammer's head or the clawed back (the clawed back is probably more powerful to kill a zombie from behind for a unseen deadly strike)