Google has introduced face recognition in Picasa web album. Now you can add name tag to faces in your pictures. Picasa web album will help you to automatically recognize faces in the picture which you can easily tag.
Gmail contact list will help you to auto-complete the name while tagging. The new contact that you will make in Picasa web album will automatically become available in gmail contact list.
It took me around 20 minutes to tag around 200 pictures. It provides a bunch of similar faces to tag at once. You can select or deselect images while tagging.
