Bundler - Bundler maintains a consistent environment for ruby applications. It tracks an application's code and the rubygems it needs to run, so that an application will always have the exact gems (and versions) that it needs to run.
rabl - General ruby templating with json, bson, xml, plist and msgpack support
Unicorn - Unicorn is an HTTP server for Rack applications designed to only serve fast clients on low-latency, high-bandwidth connections and take advantage of features in Unix/Unix-like kernels.
SimpleCov - SimpleCov is a code coverage analysis tool for Ruby 1.9.
Zeus - Zeus preloads your Rails app so that your normal development tasks such as console, server, generate, and specs/tests take less than one second.
factory_girl - factory_girl is a fixtures replacement with a straightforward definition syntax, support for multiple build strategies (saved instances, unsaved instances, attribute hashes, and stubbed objects), and support for multiple factories for the same class (user, admin_user, and so on), including factory inheritance.
Sass - This gem provides official integration for Ruby on Rails projects with the Sass stylesheet language.
ActiveAdmin - Ruby on Rails plugin for generating administration style interfaces
Delayed::Job - Database based asynchronous priority queue system -- Extracted from Shopify
Resque - Redis-backed Ruby library for creating background jobs, placing them on multiple queues, and processing them later.
Tools and Services
Heroku - Hosting provider for Ruby, Node.js, Clojure, Java, Python and Scala
Forward - Forward your localhost to a publicly-accessible url
Mandrill - Transactional email based on MailChimp infrastructure
Jenkins - Open source continuous integration server
Ruby on Rails Guides - These guides are designed to make you immediately productive with Rails, and to help you understand how all of the pieces fit together.
Better Specs - Collection of "best practices" that other developers learn through years of experience.