If you're on Windows or Linux, give Radrails a try...http://www.radrails.org. I have had good luck with it, but it is a little heavier than just an editor. It's built on the Eclipse platform.
there’s also a color scheme [1] for RadRails that is similar to Black Pearl [2] theme for Textmate
[1] http://drnicwilliams.com/2006/08/08/textmate-theme-for-radrails/
vim has some nice plugins for rails, take a look at the wiki: http://wiki.rubyonrails.org/rails/pages/HowtoUseVimWithRails
version 7 can give you some completion functions
