One of my favorite databases on photography is Luminous Lint. It is unlike so many other databases, in that it is searchable in so many different ways- by photographer, technique, theme, date, geography... it's just amazing. It contains not only images, but short bios of photographers, and has lots of galleries that allow you to view these photographs in different contexts. It also contains images that you would rarely see in galleries or museums, plus articles on unusual topics such as non-canonical photography, sequences and series, and fabricated realities.
The site's creator, Alan Griffiths, is completely enamored with photography, and it shows! It is a fantastic research tool.