The best open source IDE for developing ruby and rails programs in my opinion is the netbeans IDE version 6 and the upcoming version 6.1. It has numerous support features which you can view at the netbeans website
The next IDE I would vote for is the Aptana studio. This comes at a cost for additional features but they have really revamped the radrails plugin to a really cool IDE which you can also plug in to your Eclipse.
There are a number of other commercial IDE for ruby and rails.