Following up on my last post about Ruby gems, here's a list of some of my typical choices for various tasks in the Ruby and Ruby on Rails world:
active_model_serializers
Makes it real easy to define exactly how to serialize models in an API
annotate
Annotates Rails/ActiveRecord Models, routes, fixtures, and others based on the database schema. Very cool to know exactly which fields are present in your models, without having to look it up in the schema.rb file.
awesome_print
Great Ruby dubugging companion: pretty print Ruby objects to visualize their structure. Supports custom object formatting via plugins.
aws-sdk
The Amazon Web Services SDK allows you use those services with ease. Especially great as a backend for paperclip attachments.
brakeman
Brakeman detects security vulnerabilities in Ruby on Rails applications via static analysis. Just install and run it at your RoR root folder.
bugsnag
A Ruby notifier for bugsnag.com. It allows you to receive debug emails when stuff is broken in production. Priceless.
cancancan
Simple authorization solution for Rails. All permissions are stored in a single location. Allows you to define who has access to what in your RoR in a very simple way.
chosen-rails
Chosen is a javascript library of select box enhancer for jQuery and Protoype. This gem integrates Chosen with Rails asset pipeline for easy of use.
ckeditor_rails
This gem provides CKEditor driver for your Rails 3 application. CKEditor is the the fastest and easiest way to integrate a rich text editor into your application.
devise
Flexible authentication solution for Rails with Warden. Extremely modular way of authenticating users in your app.
devise-i18n
Everyone knows devise, but not everyone knows there is a gem with translations (i18n) for most languages.
factory_girl
factory_girl provides a framework and DSL for defining and using factories - less error-prone, more explicit, and all-around easier to work with than fixtures. Basically allows you to create model instances for your tests in a very easy manner.
factory_girl_rails
factory_girl_rails provides integration between factory_girl and rails 3 or newer (currently just automatic factory definition loading). The simplest way to start using factory_girl in a RoR app.
figaro
Simple, Heroku-friendly Rails app configuration using ENV and a single YAML file. Another way to manage env variables in a cross environment way would be rbenv-vars. But figaro also works really well.
foreman
Process manager for applications with multiple components. Just create a Procfile with the processes you wish to start and fire foreman. It will dump the logs in different colors per process to make it easier to read.
httparty
Makes http fun! Also, makes consuming restful web services dead easy. Allows you to make a REST call to wherever and get the results properly parsed into a Ruby hash, independently of the response format (XML, JSON, etc).
i18n-js
It's a small library to provide the Rails I18n translations on the Javascript. No more interpolation from slim, haml, or erb for translations. Just use plain javascript.
js-routes
Generates javascript file that defines all Rails named routes as javascript helpers. Same as above but for rails routes instead of localizations.
leaflet-rails
This gem provides the leaflet.js map display library for your Rails 4/5 application. Leaflet is an awesome javascript abstraction to handle a map on a webpage, agnostic to the map provider itself. You can use OpenstreetMaps, Mapbox, Bing and other map tile providers.
materialize-rails
Materialize (http://materializecss.com) set up for the Rails 4+ asset pipeline. Beautiful Material Design by Google at your fingertips.
mina
Do you know Capistrano? Meet Mina, a blazing fast application deployment tool. It opens only one ssh connection for all commands, so it's a gazillion times faster than good old cap.
nested_form_fields
Rails gem for dynamically adding and removing nested has_many association fields in a form. Uses jQuery and supports multiple nesting levels.
newrelic_rpm
New Relic is a performance management system, developed by New Relic, Inc (http://www.newrelic.com). New Relic provides you with deep information about the performance of your web application as it runs in production. They have a free tier so you can keep an eye in your web app in terms of performance without any cost.
nokogiri
Nokogiri (鋸) is an HTML, XML, SAX, and Reader parser. Among Nokogiri's many features is the ability to search documents via XPath or CSS3 selectors. If you need to do any XML manipulation, or HTML scraping, this is your go to tool.
oj
The fastest JSON parser and object serializer. Just drop it into any Rails application and get the instance performance increase in JSON handling.
opencage-geocoder
Need to do some geocoding or reverse geocoding? Opencage has a free tier. This gem gives you the client to interact with their services.
paperclip
Everyone knows the most common and easy upload management for ActiveRecord. Need an attachment on your models? Done.
ransack
Ransack is the successor to the MetaSearch gem. It improves and expands upon MetaSearch's functionality, but does not have a 100%-compatible API. It basically helps you build search forms for models in an easy way.
react_on_rails
With webpacker you can already get React integration, but React on Rails can give you additional features, like:
- Server rendering, often for SEO optimization.
- Easy passing of props directly from your Rails view to your React components rather than having your Rails view load and then make a separate request to your API.
- Redux and React-Router integration
- Localization support
- Rspec test helpers to ensure your Webpack bundles are ready for tests
ribbonit
Display an elegant Github like ribbon to your Rails application showing environment informations (Rails environment, Ruby version, Rails version, ...). For example: 
rspec-html-matchers
Nokogiri based 'have_tag' and 'with_tag' matchers for rspec 3. Does not depend on assert_select matcher, provides useful error messages. Makes it a breeze to test for presence of certain tags in an HTML response body.
sidekiq
One of the simplest, most efficient background processing tools for Ruby.
sidekiq-cron
Enables you to setup sidekiq background jobs to be run at a specified time (using CRON notation).
slim
Slim is a template language whose goal is reduce the syntax to the essential parts without becoming cryptic. Do you hate ERB's spaghetti syntax? So do I. Slim is a more Ruby'esc template system, based on indentation.
spreadsheet
The Spreadsheet Library is designed to read and write Spreadsheet Documents. As of version 0.6.0, only Microsoft Excel compatible spreadsheets are supported. Spreadsheet is a combination/complete rewrite of the Spreadsheet::Excel Library by Daniel J. Berger and the ParseExcel Library by Hannes Wyss. Spreadsheet can read, write and modify Spreadsheet Documents.
stamp
Oldie but goldie, Stamp formats dates and times based on human-friendly examples, not arcane strftime directives.
thinreports
Need to do some reporting and don't want the hassle of handling XML to pass to Jasper Reports? Try ThinReports, an open source report generation tool for Ruby.
validates_timeliness
Adds validation methods to ActiveModel for validating dates and times. Works with multiple ORMS.
will_paginate
will_paginate provides a simple API for performing paginated queries with Active Record, DataMapper and Sequel, and includes helpers for rendering pagination links in Rails, Sinatra and Merb web apps.
dropzonejs-rails
Adds Dropzone, a great JS File upload by Matias Meno, to the Rails Asset pipeline. DropzoneJS is an open source library that provides drag’n’drop file uploads with image previews. It’s lightweight, doesn’t depend on any other library (like jQuery) and is highly customizable.
