One thing that has always defined Joker in DC comics, movies, and TV shows, is that he is not a good person. He is someone who doesn't care about anyone or anything and only exists to cause trouble, hurt people, and create anarchy. He is the antithesis of Batman in almost every conceivable way. However, there are rare cases where Joker does something heroic.
While Joker mostly exists to prove that there is evil in everyone when he does something good, there is often an excellent reason and it is often shocking to see. With that said it does happen on occasion, and Ranker has a good listing of those rare moments where Joker steps over the line of a despicable villain to become a hero.
Batman: White Knight
In the Elseworlds story Batman: White Knight, Joker receives a cure thanks to a new mixture of drugs and becomes a good guy named Jack Napier. He uses this second chance to try to make a difference when he becomes a Gotham City council member. He targets the corrupt Gotham City police force, as he tries to make a difference.
This story even sees a second Joker arrive in the New Joker, and Jack teams up with Batman to attempt to bring this new villain down. As expected with a Joker turns good story, this one doesn't end well for the Clown Prince of Crime, but he does do more good than bad by fixing problems he caused before his temporary cure.
Going Sane
There is a mode of thought that theorizes that if there was no Batman, there would be no Joker. This was put to the test in Batman: Going Sane. In this story, Joker trapped Batman in an explosion and believed that his enemy died. After this, Joker realized Batman was gone and immediately changed.
Joker became Joseph Kerr and began living a peaceful life as a normal citizen, no longer committing any crimes. This was a story that showed how Joker could be a normal citizen, but it was the thrill of fighting Batman that kept him on the wrong side of the law for years.
The Dark Knight Returns
The Dark Knight Returns starts with Batman in retirement. He chose to stop his crime-fighting and lived in isolation in Wayne Manor. This had an unexpected side effect. When Batman retired, Joker no longer had an enemy to fight. Much like Going Sane, with Batman gone, Joker stopped committing crimes as well.
Things were different here, though. Joker turned himself in and accepted his imprisonment at Arkham Asylum, choosing not to escape, as he had no reason to return to his life of crime. Once again, Joker did the right thing when Batman was gone, furthering the argument that Batman almost creates his villains in Gotham City.
Batman R.I.P.
When Joker does good things, it is often for his own selfish and personal reasons. In Batman R.I.P., there was a new gang of villains terrorizing Gotham City, and Joker has never played well with others. This was the Black Glove and they wanted to kill Batman and tear down everything he represented.
There are plenty of villains that Joker has worked with, but when it comes to killing Batman, he has always believed he was the only one who deserved that honor. Because of this, he betrays the Black Glove, kills several of them, and leads them into a trap where Nightwing, Robin, and more capture the leaders.
Dark Knights: Metal
If there is one thing that can turn even the Joker into a hero, it is a worse version of Joker. This happened when The Batman Who Laughs showed up in Batman's world. In Dark Knights: Metal, Batman had to face an evil version of himself, a variant of Batman who inhaled Joker's toxin gas in his world and became a combination of Batman and Joker.
Since this Batman knew everything about Bruce Wayne, and also had the failsafe devices to beat all DC's heroes, no one could stop him. However, that is where Joker comes in. There is no rhyme or reason to Joker's actions, and by teaming with the real Batman, this was the only way to beat the evil Batman Who Laughs.
Countdown
When it comes to alternate Earths, there is no telling how the heroes and villains will change. An example of this is the Earth where the Justice League became DC villains known as the Crime Syndicate and devastated that Earth. On the Earth where the Crime Syndicate ruled, Joker was a hero.
On Earth-3, Joker was called the Jokester, and his main enemy was Owlman, which was that Earth's version of Batman. This version of Joker is the greatest hero on Earth, but he didn't stand a chance when the Crime Syndicate finally ed forces.
Superman: Distant Fires
Superman: Distant Fires is another Elseworlds story, this one showing Earth after an apocalyptic nuclear war destroys much of the planet and strips most superheroes of their powers. The war also helped Joker turn over a new leaf and become a productive member of the community of survivors attempting a rebuild.
Joker leads his small community as a figurehead who fights to keep everyone safe and protected. This is a rare look at a Joker who actively works to do good for no reason other than that it is the right thing to do, one of Joker's most wholesome moments in comics.
Catwoman Annual #3
Catwoman Annual #3 is another alternate reality story and takes place in a world where Batman and Catwoman are a married couple. However, instead of a crime-fighting duo, they are a criminal duo and run rampant through the city.
That brings up Joker, who in this story is named Jock Bozer. He is scarred, thanks to Batman pushing him into a chemical vat. However, instead of becoming a criminal himself, he is a police detective who works his way up to commissioner as he fights to bring down Batman.
Joker's Millions
Joker has rarely cared about money in the comics, as his goal is simple anarchy and causing trouble every step of the way. However, there was a 1998 comic book story called Joker's Millions, based on an episode of Batman: The Animated Series, that showed that money could solve all his problems and set him straight.
In this story, a crime lord dies and in his will, he gives $250 million to Joker. With all the money he could ever want, he stops being a villain, buys a mansion, and starts playing golf. He also helps the people in Gotham City by throwing money into the street as he drives past them.
Batman: Cacophony
Doing something heroic doesn't always mean fighting bad guys or giving money to those in need. Sometimes, it can be the little things that a person says or does. In Batman: Cacophony, Joker almost dies and is taken to Arkham Asylum to recover.
It is at this time that he regains his mental awareness. He is no longer a threat and Bruce Wayne shows up to talk to him, wanting to make sure that this isn't a trick, the two have a rational conversation for the first time in their lives with each other. It is at this time that Joker lets Bruce know that he is sorry about Wayne losing his parents as a child, one of the only humane things Joker has ever said.
