It is a truth universally acknowledged that Flickr has a load of APIs. Recently, a website named Polar Rose has emerged, which offers to run face recognition over your Flickr photos.
Note that I'm not calling it face detection, which is merely being able to find a face, but face recognition -- it should be able to know who the face is. This is quite a tough problem: I remember coding face detection at University wasn't that easy.
So, I added my photos and gave it a go. There's a processing time delay as it sucks everything out of Flickr (they estimated 30 mins for my 700 odd photos). After it's done, you get to start naming people: it pops up a face, asking who it is. When you tell it, it shows you similar faces and asks if these are also the same person.
My experience started smoothly. The first picture it chose was one of me, wearing sunglasses. Adding my name was easy enough, but then the subsequent pictures were all of my girlfriend, also wearing sunglasses. Not a good start overall.
As I kept tagging faces, I was generally impressed by the face detection. It seemed to be picking most faces, even partly obscured ones. I was less enamoured by the recognition, which seemed to find at most maybe two other photos of the same person.
Eventually though, it all got a bit silly. It kept asking me to identify people I had already said I didn't want to, and started showing faces that weren't even faces. I suggest the threshold for what is considered a face might need to be adjusted a bit higher, as by the end I was getting almost an even ratio between faces and non-faces.
Although I read it would tag my original Flickr photos with the names of the people I'd chosen, I checked the originals and couldn't see any. While I can understand this -- if you can browse pictures of people through tags on Flickr, why would you use Polar Rose -- it's a little bit sad in this era of openness.
Overall, I'd recommend Polar Rose as an entertaining way to spend a rainy afternoon. I don't think I'll be coming back frequently though; the face recognition gimmick quickly wears thin, and its accuracy deteriorates quite rapidly.
Update: if I was friends with William Shatner, I'd be in more luck.