Rails Plugins
This was brought up at #rubyonrails -- someone's idea of the top 11 Rails plugins.
There are a few missing from there:
Technoweenie (Rick Olsen)'s acts_as_authenticated and restful_authentication. These plugins do not tell you how to authenticate anything. Rather, you install the plugin then generate authentication code. From there, you modify it as you see fit.
Technoweenie also wrote attachment_fu
Rather than acts_as_taggable, I use acts_as_taggable_on_steroids, which is based on acts_as_taggable. Neither supports multiple tags per model, and the acts_as_taggable_on_steroids author invited me to write a patch, should I write one.
The largest repository of Rails plugins I've found is located at http://agilewebdevelopment.com/plugins.