According to the Wall Street Journal (link apparently broken) and ABC News and another link to the NYT, which have obtained independent confirmation. Here's the Times story, since the WSJ link is dead:

The actor Philip Seymour Hoffman was found dead in his New York apartment on Sunday morning of an apparent drug overdose, according to a law enforcement official who requested anonymity because he was not certain the actor's family had been informed of the death.

The official said Mr. Hoffman, 46, had been found in his West Village apartment around 11:30 a.m. by a friend who had become concerned at not being able to reach Mr. Hoffman.

A cause of death had not yet been determined, the official said, but it appeared to have been the result of an overdose.

A woman answering the phone at Mr. Hoffman's New York production company declined to comment. "I don't have a comment at this time on that, thank you," she said.

Edited to Add:

I think Synechdoche, New York might be my favorite PSH performance. Such an amazing little film. But he did great work in so many movies - Almost Famous, Boogie Nights, The Master, Before The Devil Knows You're Dead - hell, he helped lift Catching Fire from the fun movie Hunger Games was to an inspired level of genre fun.

Edited Again to Add

There's a great Twitter hashtag going right now of #PSHBestScenes