In know some of our members are using Google Picasa to organize their photos (in fact, we even teach a course on how to do that). If you are one one those, a post on the Ancestor Search Blog may be of interest.
In the post you are given an explanation of face recognition software included in Google Picasa Web Albums. Essentially, if you post photos in the web album, Google's software can scan those photos and, using its face recognition software, pick out the photos that have the same people in them. A little thought can reveal how this can help identify unknown people is some photos.
If that is not all clear, read the blog where the author explains her use of the software. The post also has attached to it some comments from readers where they tell of their own experiences. You can read the blog at: http://ancestorsearch.blogspot.com/2009/08/how-google-picasa-face-recognition.html