Some Rails plugins I find useful:

 * <a href="http://matthewman.net/articles/2006/09/04/new-rails-feature-simply_helpful">Simply Helpful</a>
 * <a href="http://svn.depixelate.com/plugins/focus_fu/">Focus-fu</a>
 * <a href="http://require.errtheblog.com/plugins/browser/sexy_migrations/README">Sexy Migrations</a>
 * <a href="http://blog.inquirylabs.com/2006/09/28/textmate-footnotes-v16-released/">TextMate Footnotes</a> (just awesome)
 * <a href="http://agilewebdevelopment.com/plugins/acts_as_taggable_on_steroids">acts\_as\_taggable\_on\_steroids</a>
 * <a href="http://errtheblog.com/post/14">acts\_as\_textiled</a>