Ledger's fame will always be tied to "Brokeback Mountain," the tender film of two cowboys and their tragic love.
They separated last year.
Before his relationship with Williams, Ledger had highly publicized relationships with actresses Heather Graham and Naomi Watts. He met Watts while working on "The Lords of Dogtown," about the growth of skateboarding in Santa Monica and Venice.
Ledger will appear as the Joker this year in "The Dark Knight," a sequel to 2005's "Batman Begins."
"The Dark Knight" is one of the most anticipated movies of 2008, and for months Batman fans have been debating the film's biggest wild card: the choice of Ledger as the Joker, a role that comes with the considerable challenge of filling the clown shoes so memorably worn by Jack Nicholson in 1989.
"The studio is stunned and devastated by this tragic news," said Alan Horn, president and chief operating officer of Warner Bros., and Jeff Robinov, president of Warner Bros. Pictures Group, in a statement. Warner produced "The Dark Knight."
"The entertainment community has lost an enormous talent. Heath was a brilliant actor and an exceptional person. Our hearts go out to his family and friends," they said.
Ledger also had starring roles in "A Knight's Tale" and "The Patriot" and played the suicidal son of Billy Bob Thornton in "Monster's Ball."
His latest film role was in "I'm Not There," in which he played one of the many incarnations of Bob Dylan. Also in the film was Cate Blanchett, whose performance in it earned her a supporting-actress Oscar nomination today.
Gold and Lieberman reported from New York. Times staff writers Michael Muskal, Jesus Sanchez and Geoff Boucher contributed from Los Angeles.