Jackson Pollock, 1948. Sold privately in 2006 for $140M — among the most expensive paintings ever changed hands.
From Wikipedia
No. 5, 1948 is a 1948 painting by Jackson Pollock, an American painter known for his contributions to the abstract expressionist movement. It was sold on 22 May 2006 for $140 million, a new mark for highest ever price for a painting, not surpassed until April 2011.