For those that don’t know (uhh, that would be most people), del.icio.us is a social bookmarking system for storing your personal bookmarks according to a set of tags. It’s quite easy to use, and a lot of people are using it. It also aggregates everyone’s tags into a cohesive, searchable database. By the very nature of the way people use it, it tends to be a great way to track where the action is on the web for particular topics.
Anyhoo, someone at hubmeg.org has written the most awesome visualisation tool for del.icio.us which links together sets of related tags into a kind of stretchy web thing that just has to be seen to be believed.