Major dependencies that I use on Grunt, Gulp, Bower, Karma and Node/CLI.
- active_admin - With ActiveAdmin the creation of admin interface for your Rails app is child's play. You get a nice dashboard, CRUD UI and lots more. Very flexible and customizable.
- better_errors - Better Errors replaces the standard Rails error page with a much better and more useful error page. It is also usable outside of Rails in any Rack app as Rack middleware.
- bullet - The Bullet gem is designed to help you increase your application’s performance by reducing the number of queries it makes. It will watch your queries while you develop your application and notify you when you should add eager loading (N+1 queries), when you’re using eager loading that isn’t necessary and when you should use counter cache.
- cancan - CanCan is an authorization gem that lets you restrict users access to resources. All permissions are defined in a single file (ability.rb) and convenient methods for checking and ensuring permissions are available throughout the application.
- capybara - Capybara aims to simplify the process of integration testing Rack applications, such as Rails, Sinatra or Merb. Capybara simulates how a real user would interact with a web application. It is agnostic about the driver running your tests and currently comes with Rack::Test and Selenium support built in. HtmlUnit, WebKit and env.js are supported through external gems. Works great in combination with RSpec & Cucumber.
- carrierwave - the ultimate file upload solution for Rails. Support both local and cloud storage for the uploaded files (and many other cool things). Integrates great with ImageMagick for image post-processing.
- compass - Great gem that adds support for some css frameworks. Includes collection of sass mixins that reduces code of css files and help fight with browser incompatibilities.
- devise - Devise is full-featured authentication solution for Rails applications. In most cases it's preferable to use devise to rolling your own custom authentication solution.
- fabrication - a great fixture replacement (editor's choice).
- factory_girl - an alternative to fabrication. Nice and mature fixture replacement. Spiritual ancestor of fabrication.
- ffaker - handy gem to generate dummy data (names, addresses, etc).
- feedzirra - Very fast and flexible RSS/Atom feed parser.
- friendly_id - Allows creation of human-readable URLs by using some descriptive attribute of the model instead of its id.
- globalize - Rails I18n de-facto standard library for ActiveRecord model/data translation. Globalize for Rails and is targeted at ActiveRecord version 4.x. It is compatible with and builds on the new I18n API in Ruby on Rails and adds model translations to ActiveRecord. For ActiveRecord 3.x users, check on the 3-0-stable branch.
- guard - fantastic gem that monitors file changes and invokes tasks based on them. Loaded with lots of useful extension. Far superior to autotest and watchr.
- haml-rails - haml-rails provides Rails integration for Haml.
- haml - HAML is a concise templating language, considered by many (including yours truly) to be far superior to Erb.
- kaminari - Great paginating solution.
- machinist - Fixtures aren't fun. Machinist is.
- rspec-rails - RSpec is a replacement for Test::MiniTest. I cannot recommend highly enough RSpec. rspec-rails provides Rails integration for RSpec.
- sidekiq - Sidekiq is probably the easiest and most scalable way to run background jobs in your Rails app.
- simple_form - once you've used simple_form (or formtastic) you'll never want to hear about Rails's default forms. It has a great DSL for building forms and no opinion on markup.
- simplecov-rcov - RCov formatter for SimpleCov. Useful if you're trying to use SimpleCov with the Hudson contininous integration server.
- simplecov - code coverage tool. Unlike RCov it's fully compatible with Ruby 1.9. Generates great reports. Must have!
- spork - A DRb server for testing frameworks (RSpec / Cucumber currently) that forks before each run to ensure a clean testing state. Simply put it preloads a lot of test environment and as consequence the startup time of your tests in greatly decreased. Absolute must have!
- sunspot - SOLR powered full-text search engine.
This list is not exhaustive and other gems might be added to it along the road. All of the gems on the list are field tested, have active development and community and are known to be of good code quality.