Tareq Iqbal has written a great overview and walkthrough of using the popular Eclipse IDE for Ruby development. Eclipse is one of the most popular IDEs currently used and many Java developers will already be familiar with it. Its cross-platform support makes it an ideal environment for development teams working across different architectures. Eclipse also has strong plug-in support and a number of Ruby plugins are available.
Iqbal walks through the installation of Eclipse before moving on to installing and configuring the Ruby plugin. Screenshots are included for each of the major steps, making this an ideal 'get up and running' guide, particularly if you want to show other developers how to set up Eclipse.