Crowbar

When Joker tricked Robin (Jason Todd) to fall into his trap, his weapon of choice in dealing with him was a crowbar. Joker brutally beat Jason Todd to near death.   
 
When Jason Todd returned from the "dead" and took on the guise of the Red Hood (Joker's first identity), he eventually confronted Joker and beat him with a crowbar but let him live.
 
After Batman's "death," Jason Todd assumed the guise of a violent Batman. When Tim Drake confronted him, a crowbar came into the mix. 
 
When Joker was in police custody, the current Robin, Damian Wayne, pulled out a crowbar to give Joker a taste of what he had delivered before. 
 
When the Dealer's Black Mirror auction practice caught Batman's attention, the crowbar was one of the items available for Gotham's (sick) elite to bid on. The description given was that it came from Lebanon, Berut and was a former possession of the Joker. Dick grabbed the crowbar when leaving and it was later recovered by Alfred in an alley. Dick ordered Alfred to simply toss it in the river.