The Great Buildings Collection provides a "gateway to architecture around the world and across history documents a thousand buildings and hundreds of leading architects, with 3D models, photographic images and architectural drawings, commentaries, bibliographies, web links, and more, for famous designers and structures of all kinds."
The Historic American Buildings Survey (HABS), from the Library of Congress, documents achievements in architecture, engineering, and design in the United States and its territories through a comprehensive range of building types and engineering technologies. The database is searchable by keyword, subject, and location. Warning: only about 25% of the records for the buildings are complete.
Funded by the William Penn Foundation, the Philadelphia Architects and Buildings Project (PAB) will bring together the resources of the Athenaeum of Philadelphia, the University of Pennsylvania Architectural Archives, the Philadelphia Historical Commission, and the Pennsylvania Historical and Museum Commission. The database is searchable by project, address, architect, and institution.
ArchInform includes more than "9000 built and unrealized projects from various architects and planners" and focuses on 20th century architecture. Be careful because some of the information is coming from Wikipedia.