Application specific plugins, although useful, are limited in their user base. If you really want to contribute to a community, create something that everyone can use.
Jonathan Snook, one of the newest members of the 9rules Network, has written a great article on adding tags to your site using PHP and mySQL. His method is not limited to any specific CMS, which makes it relatively easy for anyone wishing to use tags on their site to do so. This type of contribution is far more reaching than developing a plugin for any specific app.
If you are not sure why someone would want to use tags over categories, check out the Wikipedia article on Tags. Also, for great examples of Websites that use tagging see: Technorati, Flickr and del.icio.us. All of which are thriving folksonomies.
I personally look forward to Jonathan’s future in the Network. Check out his archives for other great finds.