The jEdit Ruby Editor Plugin (oops, this link doesn’t exist anymore. Maybe http://plugins.jedit.org/plugins/?RubyPlugin is an alternative) adds Ruby support to the jEdit programmer’s text editor. It provides method completion for the Ruby core types and an interactive Ruby documentation viewer, which is immensely useful for beginners. It is released as free software under the GPL license.
see RubyDevelopmentTools for more info about setting up jEdit for Ruby on Rails.