My text editor of choice for PHP work is Panic's Coda. But for Ruby on Rails, its autocomplete feature is continuously in my way and ROR's standardization on 2 space tabs does not play well with Coda's settings. I've been going back to TextMate for ROR work, but was wondering if there were better options.
You could try Aptana RadRails, which is free and based on Eclipse (although it seems slightly more stable for pure RoR than Eclipse-with-RadRails-plugin. I've only used it on Windows, but it does seem to have a Mac download available.
A lot of Ruby on rails developers use TextMate, you can see a lot of in several screencast. But there are a lot of use of MacVim too.
In this twice editor, there are a lot of plugin to help you coding with rails.
In IDE, there are Netbeans too.
If you think it's worth to give it a try, in my opinion TextMate is the best but it's not free.
The integrated boundles work very well for Ruby and Ruby On Rails.
There are also boundles for Subversion/Git/etc. which can be very usefull in development.
I'd recommend using Textmate, it has nice bundles for Rails and offers a great interface. There's even a peepcode episode about it.
I wouldn't use an IDE for Rails, but if you really want to, both Netbean and Aptana are worth looking at.
I use textmate and I love it. I recommend installing ProjectPlus plugin that adds svn and git support to textmate drawer ( http://ciaranwal.sh/2008/08/05/textmate-plug-in-projectplus ).
If you are searching for full IDE NetBeans looks very good: (http://netbeans.org/features/ruby/index.html)
+1 for Textmate and Command Line
I used Eclipse/Radrails for a year and it totally sucks.