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USC's Digital Archive includes tens of thousands of old Southern California photos like these. Whatever your area of interest, they've got something to amaze and amuse. Do, however, watch out for the conditions of use, which generally involve money changing hands. (I'm hoping I don't get a big bill just for promoting their site like this.) Thanks to Don Ballard for calling my attention to this amazing repository of information.
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On a far less massive scale, but still of interest, both the Orange County Archives photostream and the Orange County History image pool continue to expand on Flickr. It's worth checking in from time to time and seeing what's new on both.