Genealogy Today's Genealogy Photos web page provides an indexed search of collected photos, along with articles on copying, identifying, and preserving family photographs. Links are also provided to other sites containing old photos, some with identifying information and some with no information at all.
One of the more popular sites for placing and researching old photos is Dead Fred's Genealogy Photo Archive at deadfred.com. The site is free and users can post photos and research the collection. Other sites include, Ancestry Genealogy Photo Archive; Family Old Photos; and Ancient Faces.
Many additional web sites carry unidentified photos and may accept your unidentified photos or photos of photos you have acquired that you wish to place with related families. Cyndi's List of Photographs & Memories, Lost & Found provides links to many such web sites, although (at this writing) several of the links are broken or no longer valid.
Thousands of family photos are being uploaded to Ancestry.com's Public Member Photos, but access is limited. Non-members can do a search and view thumbnails for free, but you must be an Ancestry member to bring up any related information on the photo or its submitter. Because large numbers of people submitting to the site, It may be a good finding tool. Some local libraries offer free access to Ancestry.
The popular Flickr web site has a growing genealogy community; you must be registered to join, but Flickr is part of Yahoo and registration is free. Although Flickr is not a "lost photos" web site, as more and more photos are uploaded to the site, it's possible you may find a connection. The same can be said of various genealogy communities where family information is being shared with the aim of connecting to others, including various message board sites and social networks. And it's not a bad idea to check Google (and be sure to check Google Images), you might find photos of an ancestor buried in a web site of which you were not aware. Also, checking different search engines may bring up different results or the same results with a different priority.
Finally, most of us have photos of family friends or other unrelated people, which are also good to share, especially photos taken before the current digital age. It has been said that today's children are the most documented in history, but there was a time when cameras were not as plentiful and the cost of film and developing precluded taking a lot of pictures. And like old photos, most people have acquired bits and pieces of memorabilia or ephemera -- marriage and birth announcements, funeral programs, old business cards, membership lists, etc. Anything that pinpoints a person or persons in time in place has a potential value worth sharing.
Above all, do not discard unidentified or unrelated photos and memorabilia, but take the time sift and sort, then consider donating to a local library, genealogical society, or archive, or upload to one of the several web sites accepting such material. And be sure to check from time to time for your own family names — the unexpected find of an ancestral photo or loved one is a genuine treasure, and it doesn't always come through family channels.