The Thomas Jefferson building of the Library of Congress is just across the street from the Capitol building in Washington DC. It may be the most beautiful building in the world. Completed in 1898, it is covered in sculptures and murals portraying gilded age ideals; one section of painting shows personifications of the various scientific disciplines from astronomy to biology.
If you are a fan of art nouveau works, as I am, the Thomas Jefferson building is almost overwhelming, draped from head-to-toe in exciting color and design. Below are just a few of the pictures I took. Additionally, the library houses several excellent rotating exhibits. DC has a lot of great institutions to visit, and Library of Congress should definitely be one you seek out.