ruby-mode provides basic font-locking, indentation and navigation support for Ruby source code. It was first included in Emacs 23 (previously it was a third party package, maintained by Ruby's developers). While ruby-mode is fairly barebone package it could be augmented by a multitude of minor modes such as ruby-end, ...
Ruby on Rails
Rinari stands for Rinari Is Not A Rails IDE. It is an Emacs minor mode for Rails that is actively maintained. For best results install it from the git repo.
The official documentation is alright, but for best information check out the source. It allows for 'jumps' between Model, View, Controller and their RSpec examples and Test Unit tests, as well as Cucumber steps and features.