Life Magazine was published for over 100 years, and had a strong focus on photography. Many of their photographs, from 1860 - present, are now available online. Even better, 97% of the online collection was not published in Life, and this is their first exposure to the world at large. The photographs are searchable by keyword, and many of them are tagged, allowing you to accurately locate photos from different eras, regions, and events in history. The photos are from some of history's great photographers, such as Margaret Bourke-White, Dorothea Lange, Alfred Eisenstaedt, Edward Steichen, Gordon Parks and others.
It's interesting to do searches for topics such as "Santa Fe", and see photos of a 1949 Zozobra, or less festively, two Navajo women looking at a chunk of uranium on a store counter. You can choose one of their pre-sorted subjects such as Railroads or Marie Curie. This photographic collection uses the Google search engine, and the photos are available for personal, non-commerical use.