Kaminari
helper for paginating lists (e.g. of search results) and displaying page lists (<< < ... 10 11 12 .. > >>)
Sidekiq
run jobs outside the request-response loop, so that users don't have to wait e.g. for email to send before getting their response page. Also useful for running scheduled jobs (e.g. every 3 hours)
Slim
a condensed HTML producing templating language
Carrierwave
allow users to upload files to your Rails app (and store them in Amazon S3, so that it works on Heroku)
Bullet
raise errors in development when you accidentally write an n+1 query
Stripe
get paid
ActionMailer
customize emails sent from your Rails app
Faker
quickly generate dummy (e.g. seed) data
Rubocop
get yelled at when you don't follow the style guide
ActiveModel::Serializers
an alternative way of writing JSON output
Factory Girl
reliably generate valid test data (without having to e.g. make up dummy emails and passwords)
Capybara
write user-level tests ("click on this link", "fill in this form")
SimpleCov
track how much of your code base your tests cover
VCR
record and playback API calls so that your tests aren't always using external APIs (and can run without internet access)