Eclipse is one of the most popular programming IDEs available today. Very few development environments can stand against it in terms of features or ease of use. Except for editing very small/few number of files, for which the fast responsiveness of the simple editors might be more desirable, Eclipse is becoming the most preferred development environment. Its flexible plugin system has allowed the community to work towards enhancing/adding new functionality to the core product, just like it happened with emacs. The open source license and the plugin system has aslo meant that the IDE today supports most of the popular programming languages making Eclipse far more that just a Java IDE. One such language supported by Eclipse with the support of plugins is Ruby. If you are a Ruby/Rails developer, then you must definitely give Eclipse + Ruby plugin a try.
This new tutorial by Tabrez Iqbal has a great walkthrough of setting up Eclipse and using it to develop Ruby applications along with lots of screenshots and useful information.