The National Archives is just a short walk down Capitol Hill and across the National Mall from the Library of Congress. Currently, if you live in Washington, D.C. or are visiting, there is a very interesting exhibit titled Amending America.
Each time I visit the National Archives I learn more about the great institution and new projects they are working on. The Innovation Hub, for example, provides a space for the public to scan and transcribe documents. Since my work here at the Library of Congress involves legislative materials, I enjoy browsing the records of the Center for Legislative Archives. If you are new to researching their collection, they have a helpful Getting Started With Your Research page.
If you happen to be a Member of Congress (thank you for reading In Custodia Legis), they have a page with services specifically for you.