Towering over the main part of Washington D.C. is the Washington Monument. It was designed by Robert Mills. Construction began in 1848 but was not completed until 1884. It is the tallest building in Washington D.C. proper.
Standing at 555 feet tall, it is the tallest stone structure in the world and the tallest obelisk. It held the record as the tallest structure in the world until the completion of the Eiffel Tower in 1888. It stands at roughly the middle of the Mall in Washington (how can they call it a Mall with no Gap stores?). It is a pretty impressive looking structure.
It is constructed of marble from two different places in the country (You can kind of tell from the different colors). This was because it took so long to construct. This particular shot is looking from near the Capital Building.