lol_dba is a small package of rake tasks that scan your application models and displays a list of columns that probably should be indexed. Also, it can generate .sql migration scripts.

Caxlsx_Rails provides an Caxlsx renderer so you can move all your spreadsheet code from your controller into view files. Partials are supported so you can organize any code into reusable chunks (e.g. cover sheets, common styling, etc.) You can use it with acts_as_caxlsx, placing the to_xlsx call in a view and adding ':package => xlsx_package' to the parameter list. Now you can keep your controllers thin!

Link header pagination for Rails and Grape APIs

A full-stack Facebook Graph API wrapper in Ruby.

Helpful method when you need to add some logic that figures out if the link (or more often navigation item) is selected based on the current page or other arbitrary condition

Axlsx_Rails provides an Axlsx renderer so you can move all your spreadsheet code from your controller into view files. Partials are supported so you can organize any code into reusable chunks (e.g. cover sheets, common styling, etc.) You can use it with acts_as_xlsx, placing the to_xlsx call in a view and adding ':package => xlsx_package' to the parameter list. Now you can keep your controllers thin!

Action caching for Action Pack (removed from core in Rails 4.0)

XML parameters parser for Action Pack (removed from core in Rails 4.0)

Haml-rails provides Haml generators for Rails 3. It also enables Haml as the templating engine for you, so you don't have to screw around in your own application.rb when your Gemfile already clearly indicated what templating engine you have installed. Hurrah.

Git history statistics generator

Intercom (https://www.intercom.io) is a customer relationship management and messaging tool for web app owners. This library makes it easier to use the correct javascript tracking code in your rails applications.

Translates the Rails routes of your application into the languages defined in your locale files

Allows simple search forms to be created against an AR3 model and its associations, has useful view helpers for sort links and multiparameter fields as well.

Devise Async provides an easy way to configure Devise to send its emails asynchronously using ActiveJob.

jQuery File Upload by Sebastian Tschan integrated for Rails 3.1+ Asset Pipeline

Deliver assets for Rails.

Konacha is a Rails engine that allows you to test your JavaScript with the mocha test framework and chai assertion library. It is similar to Jasmine and Evergreen, but does not attempt to be framework agnostic. By sticking with Rails, Konacha can take full advantage of features such as the asset pipeline and engines.

A Rails engine for queuing and managing maintenance tasks

Protect attributes from mass assignment

Rocket Pants adds JSON API love to Rails and ActionController, making it simpler to build API-oriented controllers.