The Chicago-based Museum of Contemporary Photography has put their collection of 8500 photographs on-line at their newly-revamped website
. The photographs don't seem to be tagged for easy searching, but you can search by words in the titles, as well as by photographer.
(Night Bathing, by Louise Dahl-Wolfe)
(Hot Shot Eastbound, Laeger, West Virginia, by O. Winston Link)
The advanced search feature is even less interesting: search by, among other things, "culture", "collector" or "place collected". Although searching for "night" in the title
does return some interesting material.