History of Washington, DC: Resource Guide and Bibliography (GWU)
Cleveland Park Historical Society
The Historical Society of Washington, D.C. and the D.C. History Center
DC Past: A curated collection of historic photographs of Washington D.C.
Library of Congress: Digital Collections
Library of Congress: Prints & Photographs Reading Room, Researching Historic Washington, D.C. Buildings (contains many research links)
Washingtoniana Collection In Special Collections (DCPL)
Planning History, National Capital Planning Commission
National Park Service, Washington, DC History Resources (needs updating)
Ghosts of DC: A Blog About the History of Washington, D.C.
Boundary Stones WETA's Local History Blog
Retropolis: The past, rediscovered (Washington Post)
Streets of Washington: Stories and Images of Historic Washington, D.C.
Washington D.C. My Hometown: The Broadmoor History Project