I think tag clouds are pretty cool, but try as I might, I haven't seen much use for them. The problem is that they're usually pretty much the same. The big words are obvious.
Tech tag clouds show "Microsoft," "Google," "Apple," etc.
News tag clouds show "Bush," "Iran," "Iraq," "US," etc.
I've never really felt compelled to click on tags in a tag cloud because the tags are either too general or too boring.
What would be cool, and perhaps someone's done this, is if when you clicked on a tag in a tag cloud, it brought you to a new, more specific tag cloud with related tags. Something LIKE that exists, called Google News Cloud. However, rather than successive mini-clouds, it simply highlights related tags.
I'd like to see something similar to the wonderful Visual Thesaurus tool.