Sure, desktop photo managers like iPhoto and, to a lesser extent, Picasa can recognise and tag faces in photos, but what about your online shots? New webapp Polar Rose provides facial recognition for Flickr accounts.
Polar Rose isn't a built-in Flickr feature, but a site that can pull in your Flickr stream and suggest Facebook contacts for tagged faces, once you authorise the site with both services. You might encounter a delay after first signing up, as Polar Rose told me I'd have about 30 minutes to wait. Once you're loaded up and signed in, the site prompts you to name or skip the faces it finds in your photos. If you name a face that Polar Rose sees elsewhere in your stream, you'll be prompted to choose which of those other photos are in fact the same person. I don't have enough actual humans in my Flickr stream to get this prompt, since I'm mostly a food porn pro, but WebWare has a screen capture.
Polar Rose doesn't seem as crafty at catching faces turned to the side or partially obscured as iPhoto, but you can manually browse through your Flickr thumbnails and hit "Edit Tags" to manually square off and tag faces. The ultimate benefit of all this tagging is that you'll get a page that organises your photos by person, and Polar Rose should be better at catching that person as they show up in future snapshots. You can choose whether your Polar Rose tagging is announced on Facebook and tagged publicly back at Flickr, which is a nice little privacy tweak.
Polar Rose is free to use, requires a sign-up and Flick authorisation (with an optional Facebook Connect login). Know another way to organise your online photos by faces or names? Tell us in the comments.