Netbeans is slowly becoming my favorite IDE. (Sorry, Eclipse!)
Today I started using the Ruby on Rails support within Netbeans, and it's quite nice. I'm probably not doing anything terribly sophisticated, but I did manage to create and run migrations, create some controllers and views, and launch the app, all from within Netbeans. Eclipse probably has support for all this stuff, but due to unexplained Eclipse crashes (on Linux), I can't actually use it.
I'm using the JRuby plugin for Netbeans, which means my rails code is actually running in Java. Kinda nice - Java is a more ubiquitous runtime environment that Ruby, so I'm thinking this will be helpful when it comes to deployment time.