A directory for heroku plugins out in the wild: <a href="http://herocutter.heroku.com" rel="nofollow">http://herocutter.heroku.com</a> and guide to writing heroku plugins: <a href="http://hone.heroku.com/heroku/2010/01/09/guide-to-heroku-plugins.html" rel="nofollow">http://hone.heroku.com/heroku/2010/01/09/guide-to-heroku-plu...</a>
The cool thing about this is -- browsing quickly through the source -- it looks like it's real easy to make your own Heroku plugins. Their addon system seems to be going gangbusters, and this is the icing on the cake ;)