RubyMine includes bundled evaluation license key for a free 30-day trial.
Ruby plugin for IntelliJ IDEA is updated according to RubyMine's Development Roadmap.
Plugin is available in the Plugin Repository. You can also download Older plugin versions.
Due to differences in RubyMine and IDEA release cycles sometimes new features will be available in RubyMine earlier than in Ruby plugin.
Ruby plugin for IDEA supports Java/Ruby integration for JRuby code, but doesn't have RubyMotion support (https://youtrack.jetbrains.com/issue/RUBY-13640).