The song Joker sings at the beginning includes several references to Joker lines from previous Batman films. "Wait's they get a load of us", echoes one of Jack Nicholson's lines in the 1989 film Batman, and "an agent of chaos" and "why so serious" are Heath Ledger's lines from the 2008 film The Dark Knight.
Commissioner Gordon (Christopher Meloni) tells Two-Face that killing a child is considered "especially heinous". This is a nod to Meloni's show Law & Order: Special Victims Unit, where the opening narration says that they investigate crimes that are considered especially heinous.
Two-Face mentions that he has to explain things twice to Gordon. This is a nod to the idea of the character and his relation with the number two, like robbing the Second Bank of Gotham or having twins as henchmen.
Joker has his Villy Awards displayed on the wall by the dinner table in Bethany's house.
The suit the Joker is wearing during the parade, the giant clown ballon, and the fact that he's throwing out money to citizens is a direct homage to the 1989 Batman movie.