Technorati has started to aggregate and combine tags from feeds (it uses categories), Flickr and Del.icio.us. Here’s an example page: Technorati: Tag: humor. They support the rel=tag attribute. It’s brilliant.
Innovation around folksonomies is going fast, a lot of it driven I think by David Weinberger, who is writing a book about tagging and taxonomies and such, and is blogging about this stuff all the time, and happens to be on the board of Technorati. I’m excited – this is great stuff. I am also starting to experiment with tags on me-tv, a videoblog aggregator project of mine.