As the former right-hand (wo)man of the Joker, Harley Quinn has a variety of trick gadgets to add the punchline to any assault. One of the most often used and easily recognized is Harley's Hammer. The Hammer has been depicted several different ways, including a wooden mallet, a spiked monstrosity, or a simple black and red wrecking tool. However, it is always comically oversized, in fitting with her circus theme. While it has been suggested that Harley's possible extra-ordinary strength strength allows her to wield her hammer, another possibility is that the hammer itself is a trick, with cleverly set weights allowing her to swing it with extras power due to centripetal force.
After DC's reboot, Harley's hammer has become slightly less comical, and more menacing, appearing as a simple wooden shaft with an enormous, vaguely wedge-shaped head of stone or metal. There is no longer any doubt that it requires enormous strength to use properly.
DC Universe Online
Harley uses her hammer in a Boss Battle against players.
Batman: Arkham City
While Harley isn't seen using her hammer, finding it in the Steel Mill is part of solving one of the Riddler's puzzles.
Injustice: Gods Among Us
Harley's hammer is used as one of her main weapons in combat. In her Super Move, she uses it to pummel her opponent, knocking him/her back.
Batman the Animated Series
Harley uses her hammer quite often within the series.
Harley Quinn toys and figurines are usually accompanied by one or several of her usual assortment of clown weaponry; including her hammer.