U.S. Capitol
The White House
Library of Congress

Washington Convention and Visitors Bureau

Jefferson Memorial
Washington Monument
Lincoln Memorial
FDR Memorial

World War II Memorial
United States Holocaust Memorial Museum
Korean War Veterans Memorial
Vietnam Veterans Memorial

The National Zoo
Smithsonian Institution
National Museum of  the American Indian (just opened in September 2004!)
National Museum of American History
National Museum of African Art
National Museum of Natural History
Folger Shakespeare Library
International Spy Museum
Ford's Theater and Lincoln Museum
The John F. Kennedy Center for the Performing Arts

Other monuments, memorials and points of interest

Sightseeing Map of the DC area
Maps of the Washington Metro System
Capitol Tour Guide Services