Listed below are high-quality websites useful for a variety of Architecture topics, but there are many others you can find on your own. Remember there are many specialized search tools you can use and also remember to carefully evaluate any information you find on the web.
Over 19,000 images of architecture and architectural sculpture of the Mediterranean Basin, a small selection and brief survey of Islamic architecture, and a searchable book on Greek and Roman Cities of Western Turkey.
The Chicago Imagebase is a Web-based project aimed at enhancing knowledge about the built environment of the Chicago region. On this site you will find a wide variety of images and other data along with information on how to use this data to study the city.
From the University of Washington Libraries. Contains nearly 10,00 images contributed from a wide range of sources and covering buildings worldwide. Images are copyrighted and may not be duplicated without permission, but "freely available to anyone with access to the Web for use in the classroom, student study, or for individual research purposes."
Structurae is an International Database and Gallery of Structures. It is provided as a free service to engineers, as well as teachers and students of structural engineering. This site contains contemporary and historical information on works of structural engineering and their designers, and firms that constructed them. The intention is to provide a broad overview of structures and as much technical information as reasonable." Also points to relevant print and web materials. Searchable; browsable by designer, geographic area, type of structure, firm, and project name.