Actually, Netbeans. Great IDE with SVN, Git support (nbgit plugin), support refactoring tools and debug mode with sweet breakpoints
Currently moving between Eclipse, Netbeans, and Textmate. At work we do both Java and Ruby development (our code base is Java with all of our acceptance tests done using rspec, so we tend to use Eclipse for both. At home I have been using Textmate, but I am trying NetBeans 6.5 out to see how it is. So far, I like it enough to replace Eclipse at work as much as I can.
I have also looked at RubyMine and Intellij and both look very promising. Only problem is the price.
I have tried Aptana Studio, Netbeans 6, RubyMine, and Komodo. After using all of these though I find myself going back to Textmate, terminal and Growl. There are some good things about each of the IDEs but they always feel bloated and sluggish and start to get in my way of being productive. If you're on a Mac you can't go wrong with Textmate, ZenTest (with autotest/autospec) working with Growl, and a term window. You can be super productive and costs you nothing.
There are lot of IDE used for RoR Development, but i like most of them are E-texteditor, Dreameweaver with addon of rubyweaver.
I work for a company that has been doing RoR development for 2 years. We are standardized on OS X for development. Among our 10 or so developers, there's an even split between TextMate and NetBeans, and one if using VIM and another is using Eclipse. As other posters have mentioned, it all has to do with what you're comfortable with (e.g. Eclipse and NetBeans users likely have been using these IDEs for a while with other languages, so it was a no-brainer to keep them for RoR development).
If you are new to development of any kind and you are using OS X, TextMate is quite a good IDE.
Ive been using NetBeans for about 1,5 years now and i like it. Im using NB with mercurial (git is supported too) and a haml-syntax-highlighter plugin.
Sometimes, when i just want to quickly edit one single file, i use Notepad++.
Linux's Gedit is also good. I use it, and customized it to look just like rails casts theme of textmate.