Bettman Archive. This is probably the largest collection of vintage images in the world, with millions of images. Recently it was bought by Corbis, a subsidiary of Microsoft.
Calisphere. This massive University of California site features visual treasures about California from the Bancroft Library and also offers links to many related California history sites.
Pier 24. An innocuous buildling along San Francisco’s waterfront contains a massive collection of historic photos. You can see the collection in person by making reservations online. Admission is free, although only a few people are allowed in the building at any one time.
Shorpy. Thousands of evocative photos from yesteryear. Photos come from all over the United States.
The UCR/California Museum of Photography features a superb historical photography collection. Contemporary photographers are also features. Many of the photographs are online.