It is reported by many news groups including the New York Times that Heath Ledger was found in Mary-Kate Olsen's Manhattan apartment; dead of a possible drug over dose. He was a father to a little girl and only 28 years old. All of the speculation in the world won't help here, which we know the media will be doing for the coming days. We, the public, are just going to have to wait for the final autopsy results come in; which we all know can take weeks to months.
Heath Ledger is best know for his roles in "Brokeback Mountain", "A Knight's Tale", "The Patriot" and his current role was going to be that of The Joker in "The Dark Night" the continuation of the "Batman" series that began with "Batman Begins".
Sources: New York Times