I've been waiting for this puppy for a while: Riya, an online service that finds photos of people based on visual matching. You tell it who's who in some of your pictures, and it finds people in the rest. The system entered private beta today, and I've been experimenting with it. It is, as the CEO notes, "so beta it hurts." There are bugs, weird disappearing training sessions, slow and incomplete recognition, and so on. But I can see it beginning to work, and it is incredibly cool. I am uploading about 3,300 vacation pictures into it, and I've trained in on a few dozen photos. Already it's helped me find some photos of friends that I had completely forgotten about.
My favorite use of the system, so far, is to enter in combinations of people to see where they've been at the same time. "Rafe Jennifer" is my favorite, since it finds pictures of me and my wife together. The service will also find text in photos--useful if you want to find people from meetings where everybody was wearing nametags. Also, you can gang up the training sessions from friends and family, which should both make it possible to find pictures of people you know in your friends' photos and help you identify strangers in your own pictures. (Since none of my friends are in on the private beta, I have not been able to test this feature.)
My sense is that, right now, the load on the Riya servers is slowing down the recognition engine pretty seriously, even though the system is still closed to the public. I do hope that the team at Riya is able to scale up the technology quickly. This is a tool everybody should have.