TagCow

Tagcow is a photo tagging service that tags users' photos with descriptive keywords.

TagCow, launched in March 2008, is a service that tags your photos with descriptive keywords. If there's a mountain in the photo, it's tagged. A dog? yep. A yellow cup? Absolutely. It does people, too. Upload an image of a person and say who it is, and all other images you upload will be tagged with that person, too. The service also integrates with Flickr and will auto tag the photos you have on the service.