Joker Venom

Joker Venom is the Joker's signature weapon.

History
When someone breathed in Joker Gas, it killed them while giving it's victim a permanent smile on his/her face. There were many variations of Joker gas, and it also affect animals, such as cats and fish.

Batman: The Animated Series
Joker venom is used frequently in the animated series, and it's spin-offs such as Batman: Mask of the Phantasm. The most popular use of it in the animated series in through a flower on the Joker's purple suit.

Batman (1989 film)
The Joker uses "Smylex" specifically in gas form, to kill everyone at a celebration in the center of Gotham.

The Dark Knight
What may or may not have been Joker Venom comes out of a grenade (in gas form) in the Manager of Gotham National Bank's mouth.

The Dark Knight (Book)
Shortly after the Joker has first met the crime lords, Sal Maroni hires a private investigator, named Hamlin, to discover who the Joker is. Several weeks later, Hamlin comes back to Maroni completely disheveled and asks if Maroni didn't just imagine Joker, because there is nothing on him, like he came out of nowhere. Then Hamlin begins laughing hysterically and dies in the middle of a restaurant. Maroni finds out Hamlin had been poisoned by some herb from out in "China, Tibet, Korea, one of those places" (he's indifferent to remembering where exactly). It's also implied in this scene that the Joker venom was derived from the same plant that the Scarecrow toxin was made from (as the Toxin was made from a Blue Poppy in Tibet).