Flickr has quietly made a big change to its tagging system, which from now on will show who added a tag to a photo or video. As Flickr's Community Manager Heather Champ notes, this information was previously available through Flickr's API, but wouldn't show up on Flickr's photo pages. With the new system you simply hover over a tag and you'll get a tool tip that shows you the username of the person who added the tag.
Since Flickr has just turned this feature on, all previous tags will simply get an "added by a Flickr Member" notation to preserve anonymity. Going forward you'll see the username, and can click on it to see other items that user has tagged.
The change, likeamendments on Tuesday, marks a trend toward having users better police themselves. With the addition of user credit on tags, there's no more anonymity when tagging since users must be signed up to make use of the feature.
It is a little surprising this took so long, since Flickr has shown credits on its on-picture notes for some time now. In practice this should give heavy Flickr taggers a little bit of a self-esteem boost, along with providing a simpler way to catch people who are using tags to spam their own shots. I wouldn't be surprised if the next step is some sort of karma system, or filtering this information into the recently overhauled activity stream pages.
One thing to note is that photo or video owners must have their privacy settings set to allow other users to add tags, something which is turned off by default. If you're looking to change that setting for all future uploads you can find it here.