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  for RadRails that is similar to Black Pearl  theme for Textmate
vim has some nice plugins for rails, take a look at the wiki:
version 7 can give you some completion functions