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.
RubyMine Extensions API
Ruby plugin for IntelliJ IDEA is updated according to RubyMine's Development Roadmap. For more details read the FAQ regarding IntelliJ IDEA Ruby Plugin vs. RubyMine.
Plugin is available in the Plugin Repository
Older plugin versions
Ruby Plugin 2.0 Release Notes
Recent changes (Updated Jun 8, 2009)
Getting latest version
Ruby Plugin 2.0 Features
IntelliJ IDEA Ruby Plugin FAQ