In a previous entry, I wrote about 43 Things and Flickr. These are two interesting, trendy, and (in least in the case of Flickr) extremely useful applications. (I think that 43 Things may be powerful in its own way, too.)

Both applications are well worth a look if you don’t know them. You use 43 Things to create a list of personal goals (here are some of mine, and a further discussion of the application). Flickr, in contrast, is used to share photos with a global community of photographers - and also for off-site image management, as in my Photoblog 2.0 and Digital Field Guide.

Flickr and 43 Things have in common that they provide a self-tagging mechanism. In 43 Things, you can apply tags you create to goals. In Flickr, you tag photos. A context in which everyone can freely tag (and categorize things) has come to be called a “folksonomy”. Put differently, a folksonomy is a bottoms-up taxonomy created by the people for the people rather than a top-down hierarchy constructed by experts - the usual model for a taxonomy.

These folksonomies are very useful for sorting, searching, categorizing, and making relevance determinations within an application. Both 43 Things, on its home page, and Flickr, on the Flickr Tags page, make use of a common visual metaphor in which the larger the font size of the tag, the more people have applied it (and the more important it is).

“Social bookmark” manager del.icio.us lets you tag and categorize web links, creating a web folksonomy competitive with web taxonomies like that of the ODP. (Technorati provides a somewhat reverse service which allows you to track usage by tag in weblogs.) So del.icio.us and Technorati have created folksonomy-related services that distribute across the myriad sites in the web.

But what about aggregating folksonomic discovery across applications (as opposed to sites)? Why shouldn’t I be able to cross-correlate 43 Things tags with Flickr tags?

A beta application named Gataga uses a frankly Google-esque user interface to aggregate social bookmark tags from del.icio.us, blogmarks, blinklist, jots, spurl, furl, simpy and connotea.

Gataga will display its folksonomic search results as an RSS feed (just as Technorati does), which is very useful: you can subscribe to stay updated. But there are big missing pieces in this application. For one thing, it doesn’t include 43 Things and Flickr, off the beaten track of social bookmarking spanning web content, but far and away my favorites for fun and utility as self-tagging folksonomies.

There’s also the issue of what you do with the folksonomic information to make it easy to grasp and genuinely useful. There has to be more than the font size = number of instances visual metaphor. I’d like to see graphic representations of similarity, relevance, occurence, and connection using dynamic link maps. This stuff has ways to go.

Still, it is a big mistake to underestimate the power of bottoms-up technology movements (witness Linux and open source). An apparently humble concept, self-tagging and the folksonomy, has the potential for toppling the hegmony of indexed search as the predominant way we find information on the web.

What are your favorite examples of self-tagging folksonomies?