I post my photos on Flickr. Like del.icio.us, there's the tag thing. And you can view the "most common tags". And this just strikes me as a very good way to suss out the best keywords for search within a given user environment. I mean, on Flickr, it becomes very clear that beach, wedding, family, and friends are going to work for people as tags. Of course there are authority problems. Some people use cat, some people use cats. That's probably the main difference between librarians and computer people. Computer people prefer to find a way around the messiness without cleaning it up: just make sure people know there's the "cat" tag and the "cats" tag. Or program it so results for both come up. Librarians have a hard time constraining the impulse to force people to choose a discrete tag. Librarians would probably ask the programmers, "can you make a little pop-up note that tells the user, 'you wanted to tag this as cat - please use the tag cats instead.' "
I like the visual Flickr uses, when you look at "popular" tags, or your own tags, or someone elses tags, it shows you which ones have the most photos associated with them by using size.
Wow, clusters. Flickr has a new thing called clusters. You check out all the photos people have tagged with "Red", then you can view clusters related to red. All the photos that were tagged "strawberry" or "autumn" in addition to "red". I am to be liking this.
UPDATE: OK, I guess I am just an idiot - no, I am just out of touch. Apparently there is actually a name for the whole phenomenon of user-created tags. Either that or Adam Rifkin coined it, but he has a delicious tag called "Folksonomies". I really need to keep up. But that's not likely to happen now that I'm a Mom.
ADDITIONAL MUSINGS: OK, so one problematic thing about these tags is how people handle phrases. Some use social_networking, some use social-networking, some use socialnetworking. So, again, there has to be a way to either bring similar tags together or exert authority control. Otherwise, valuable categorization work is lost, or subject to the vagaries of serendipity (which is not altogether a bad thing, but messy).