WorldCat: Comprehensive online catalog of library holdings. Lists books, journals, maps, recordings, films, scores, and other materials located in public, academic, corporate, school and specialized libraries all over the world. Check Worldcat for holdings of material that are not available at Penn. Available items may be requested through Interlibrary Loan.
OAIster: A subcatalog within Worldcat, OAIster is union catalog of digital resources from open archive collections representing multidisciplinary resources from more than 1,100 contributors worldwide. Records contain a digital object link allowing users access to the object in a single click.
Philadelphia City Archives: The City Archives contains deeds and mortgages from the founding of Philadelphia to 1952, birth, death, and cemetery records up to 1915, and an excellent early photograph collection, all invaluable in researching an old Philadelphia house. Its PhillyHistory site features digitized images from Archives' photography collection.
The Library Company of Philadelphia: A rare book and research library with collections documenting the history and background of American culture from the Colonial period to the Civil war.
Historical Society of Pennsylvania: The Society is an excellent source of materials on Philadelphia history, including census records, old newspaper articles, family histories, maps and atlases. The Society also has a large collection of watercolors and early photographs of various buildings and landscapes throughout the Philadelphia area.
The Athenaeum of Philadelphia: The Athenaeum holds an extensive collection of architectural plans and drawings by noted Philadelphia architects and other materials useful in researching old Philadelphia houses.
Free Library of Philadelphia: The Free Library of Philadelphia maintains an extensive collection of materials that can be used to research the history of buildings in Philadelphia. An excellent guide to their collections, as well as other resources available in Philadelphia can be found here.