Library of Congress (Thomas Jefferson Building)

It is behind the Capitol, it is free, and it has fantastic interior architecture. Everyone should visit it.
Hubby and I took advantage of being in DC without most of the children. I think they would have gotten bored a lot faster than we did.
Opened in 1897, using Jefferson's collection (that they'd bought after the British burnt the first lot that were stored in the Capitol). There are three buildings now - the John Adams and the James Madison are two other buildings, but they're not as pretty. I thought I read that the inside had been refreshed recently, but I can't find any mention of it now I look.
However, the building is in Italian-Renaissance style, and the inside was only possible because the construction engineers finished under budget, and were therefore given the extra money to make the inside pretty. And pretty it they did, although mainly from a Western point of view. Apparently even after decorating the place and putting massive doors at the entrance (they are massive...) they were $200K under budget.