if it's just for Ruby, I'd suggest jedit (www.jedit.org) with the ruby plugin
Hey… I wrote that 
Check out these downloadable packages now:
Windows version: http://www.napcs.com/products/rails/Eclipse/index.htm
Linux version (not publicly released but works on Ubuntu 6.06 and 6.10) http://www.napcsweb.com/blog/2006/11/13/eclipse-for-rails-for-linux/
Of course there’s also RadRails (www.radrails.org)