What's your favorite RoR editor/IDE?

Of course it depends on the development platform. While using Windows,
I normally have Visual Studio (with a small extension for it), but on
OS X the best editor is definitely TextMate. I wrote two short
articles about these, you can take a quick look on them here:

Using Visual Studio as a Ruby On Rails IDE
http://www.eclips3media.com/workshop/?p=8

TextMate: The best Ruby On Rails editor for OS X
http://www.eclips3media.com/workshop/?p=5

Cheers,
Attila