Flickr is something that we have talked about often, and it a program that I know little about - though I have uploaded some photos, I have yet to figure out any special features or cool tricks.
The New York Library has posted a thousand photos in the Flickr Commons and, in doing so, has significantly increased the availability of their resources.
And how beautiful they are! The NY Public Library thinks of this as "a sort of appetizer course, a sampler of collections accessible in greater breadth and depth on the NYPL Digital Gallery, and on-site in our network of libraries." There is really no greater tool at the disposal of libraries in the displaying and publicizing of their photographs.
I think that Christa is correct here: What about our little collection downstairs? No one ever takes a look at those pictures and, let's face it, we do not have the resources to preserve them for the long haul. But, if we were to digitize them and make them public some how (either with Flickr or a program on our website), we will have made them instantly usable.
By making these things available, libraries are able to reach that wider audience that we are always striving for!