As mentioned, JetBrains RubyMine and Aptana RadRails (eclipse with Ruby/RoR plugins) are great IDEs. However, I'd recommend trying Ruby (or Rails for that matter) in a text-editor like TextMate instead. Here is a *dated* article describing the pros/cons of using a text-editor vs an IDE.
During my working day, I use eclipse (corporate standard) - though my clear preference would be TextMate, which is what I use exclusively outside of work.
TextMate, or it's twin-sibling e are both "text-editors", and therefore quite different than a full featured IDE. It's a personal choice at best. I use IDEs all day at work, but greatly prefer the speed and simplicity of a text editor. JMHO
I come late to the party, just to confirm that RubyMine is really good. I still tend to be a TextMate freak, though. For a number of reasons, IDEs for Ruby are not as essential nor as powerful as IDEs for Java.
Paolo Perrotta, Europe
Author of Metaprogramming Ruby