While 3rdPartyHelpers can make it easier to re-use code for your views, libraries (contained in your rails app’s /lib directory) can extend or modify the functionality of models, controllers or just about anything in the Ruby language. Here are some libraries developed and shared by others:

• An easy TextSearch method for your models.
• A more FlexibleFixtures architecture. Rather than assume your fixture files, database tables, model class names and fixture group names all match in every case, this fixture module will let you define multiple fixtures for the same table, or give you the flexibility to use alternate fixture files in your tests.
• ActsAsPermalink
• acts_as_vertical
• TaggingWithRails lists two libraries for adding folksonomy.

• acts_as_paranoid—Doesn’t remove data when objects are deleted, but flags them as deleted instead.
• Expire Cache By Date
  Extends \ActionController::Caching to include expire_date() method for handling cache files who URLs are created using a date
• Feed Validator
  Is a Ruby interface to the W3C Feed Validation online service , based on its SOAP 1.2 support.It helps to find errors in RSS or ATOM feeds.
  Feed Validator add a new assertion (assert_valid_feed) which you can use in your functional tests in Rails applications.
• Ruby Datavision Bridge makes it easy to use the DataVision GUI report designer and then execute the XML report definition from Ruby on Rails
• Caches.rb Simple method caching library