Very recently I learned about a new plugin architecture that Rails supports which allows developers to easily develope plugins for the Rails framework. In the past, when developers wanted to introduce new functionality into Rails, they had to either create a "patch" and commit it using SVN or create a gem that you needed to download and install (much like how you install Rails itself.) The difference now is that there is a plugins folder within the vendor folder to hold all plugins and these plugins allow you to extend Rails beyond its core functionality.

I wasn't able to find any concrete documentation about this new feature (it isn't part of the stable release of Rails, but part of "Edge Rails" which can be obtained from the repository), but I did find this post by Jamis Buck (37signals team) which describes the process of Plugging into Rails.

Some publicly avaible Rails plugins announced in the Ruby on Rails mailing list:

• acts_as_enumerated, has_enumerated and ActiveRecord::VirtualEnumerations
• security_extensions and asset_timestamping
• inherits_from

Unfortunately, these plugins don't have official websites (yet), so the best way to learn about them and their usage is to try and search the Ruby on Rails mailing list archives.

UPDATE: I found the wiki page with a bunch of plugins that people have written so far. http://wiki.rubyonrails.org/rails/pages/Plugins