By popular demand, the Rails plugin directory now has tagging support.
When you view a plugin, such as acts_as_authenticated, you will see a Tags section where all the tags that have been added to the plugin are listed, as well as a box for adding your own tags. The tag box is ajax-ified, of course, with the added bonus of graceful degredation complements of respond_to.
Searching on tags is simple... just use the plugin search box. If you only want to see plugins specifically tagged with a tag, and not the ones that also match based on search, then you can specify the tag in the url, like so: http://agilewebdevelopment.com/plugins/tag/login.
But wait! There's more!
If you go to a specific tag URL like http://agilewebdevelopment.com/plugins/tag/login, you'll notice an RSS link at the bottom of the page (and auto-detected by your browser) for a feed that gets updated when plugins get tagged with that tag. In this case, that feed would be http://agilewebdevelopment.com/feeds/tag/login.
And all of this plugin goodness is free, free, free! What more could you want?