I use NetBeans 6 some at work, as it has several nice features. Frankly, though, the intellisense feature drives me batty, popping up with useless help at inappropriate times. Generally, I develop with vim and I leave the Rails console open for quick method lookups and testing.
I also agree with Matthew Vincent and Chris Blackburn that IDE and Intellisense is kinda spoiling the experience of ROR.
There's also Emacs + rinari which is a rails mode for Emacs. Emacs is a powerful text editor, cross platform that can do so much more then TextMate. There's also a peepcode tutorial for emacs to get you going.
Try Emacs with Rinari, ecb, flymake, nxhtml, yasnippets and stay tuned to http://blog.wyeworks.com. I'm going to post an article with an awesome set of plugins and an init config file for emacs.
On a Mac I tend to gravitate to TextMate, but on Linux or Windows I use RubyMine. I'm really starting to enjoy the product and I'm even finding myself using it on my Mac now too. Some things Textmate does better, but all in all it's not bad. It supports GIT and SVN, Cucumber integration, RSpec, etc. Doesn't hurt to try things and find what works for you.
Disagree with those that say "an IDE is not in the spirit of Ruby". While I've written a good bit of code in text editors, I was an IDea user for years (off and on) with Java. Even after getting well-used to Ruby and Rails using TextMate, I still want to right-click on a method to look it up and really miss code completion that worked as I typed after a short delay vs. having to do key-commands. And as time goes on, I still miss it sorely sometimes. I understand that the dynamic nature of Ruby doesn't always lend itself to that (because of use of method_missing, etc.), but that doesn't make it any less frustrating if you expect it to work that way. I've also worked with others that have worked with Rails a much longer time and can see it is a problem for them as well; they will use what is available in TextMate, but there is a lot of head scratching and searching that is wasted time, and I know for a fact someone could do something about it and still have a relatively fast and responsive experience. I still think TextMate is the best editor for Ruby though. Nothing beats doing "mate ." in the project directory to open it all up and start working immediately (or similar) and I've tried IDea for Ruby/Rails and even Aptana, but it just didn't click. I don't have the latest IDea, though.
At various times I have tried using Eclipse, NetBeans, and TextMate, with varying degrees of satisfaction. Now I'm using RubyMine from JetBrains. It's not free, but it's the best IDE I've ever used for working with Ruby and Rails based applications.
It's biggest advantage is that it pre-indexes your project, so jumping to method declarations and finding usages of methods is fast. It's CMS integration tools (it supports Subversion and Git) are top notch. And it has a highly productive testing and debugging interface. That said, it is a resource hog, so you'll want a beefy machine with lots of RAM in order to use it effectively.
Currently licenses are USD $99, but you can get a 30-day eval license, and they offer free licenses for classroom and open source project use.