JetBrains RubyMine IDE
RubyMine — is the dedicated Ruby and Rails IDE providing intelligent Ruby code editing assistance and tools for web applications development with Ruby on Rails. Find more at official web-site. IntelliJ IDEA is providing similar functionality with the Ruby Plugin installed.
IntelliJ IDEA Ruby Plugin
Plugin for IDEA 9 (Maia)
Plugin is available in the Plugin Repository.
Plugin for IDEA 8 (Diana)
You can download plugin, comment and rate it in the Plugin Repository.