National Preservation Orgs
National Trust for Historic Preservation
NPS Technical Preservation Services
(Sec. of the Interior’s Standards, Preservation Briefs, Tax incentive info)
The Association for Preservation Technology International (APT)
American Council on Historic Preservation
Library of Congress (Sanborn maps, HABS/HAER collection)
National Archives
NPS National Register of Historic Places official site

Local Preservation Orgs
DC Preservation League
Preservation Virginia
Preservation Maryland
Baltimore Heritage

Local Historic Preservation Offices (HPOs)
Maryland Historical Trust
Virginia Department of Historic Resources
DC Historic Preservation Office

Other Preservation and Historical Orgs of Interest
Historical  Society of Washington, D.C.
Maryland Historical Society
White House Historical Association
Society of Architectural Historians
docomomo US
Recent Past Preservation Network
Preservation Society of Charleston (SC)

Other Sites That I Dig!
National Building Museum
Preservation in Pink
Greenbelt Museum
Raina Regan

My Classmate Kate!
The Globe-Trotting Building Hugger

My Graduate School!
Goucher College