Dan Wellerman has written up a tutorial for NETTUTS.com about making a jQuery and PHP-powered tag cloud out of the tagging already a part of your site.
Aside from the actual links themselves, which give people an idea of the subjects that your site covers, they can also show how popular the different subjects are. Another great thing about tag-clouds is that they can be used to describe the frequency of anything; you can link to articles, blog posts, images, video, or anything else that you have in abundance on your site.
The jQuery then parses the JSON message back out and drops it into CSS styled divs already in the page. As always, the source in available for download.