We know that Dolley Madison famously saved the portrait of George Washington that hangs in the White House when the British were closing in. Were any known other major artworks or artifacts lost in the attack?

The entire library of congress was lost in the attacks and was later replaced when Thomas Jefferson sold his collection to Congress (Congress named the price and Jefferson didnt negotiate) at a price of $23,950 dollars. Though this was a wicked nice thing for Jefferson to do, he was also in perpetual and mounting debts. The original library contained about 3,000 volumes. Thomas Jefferson's personal library at Monticello held 6,707 volumes (his count), more than twice the original library. In hind sight, the painting seems pretty trivial in comparison.