I'd been using a "tag" plugin that was kind of a hack - it used its own table, and didn't integrate with Categories, so it was double duty sometimes to properly tag stuff and categorize it.
I just stumbled across the Cat2Tag plugin, which provides a fully integrated folksonomy/tagging system that uses categories to store the tags. It creates new categories if necessary, and provides a tag cloud for tag entry - now it works almost exactly like del.icio.us does.
On top of that, I also found the WordPress Heat Map plugin, which provides a tag cloud for a set of categories in a WordPress blog. I'm using it in my "Archives" page now.
Very cool combination of plugins, making a much more dynamic and organic tagging system for my blog.