You can use Kdevelop or Kate, although I do not know that there is a large community of support for these editors. If you are working on Linux, you should check out VIM, this is the 2nd most popular editor for Ruby / Rails (with Textmate - # 1).
There is a really good entry on how to configure VIM at:
http://blog.adamlowe.com/2009/12/vim-destroys-all-other-rails-editors.html
If you use VIM with all the different tpope and NerdTree plugins, you'll be pretty much tuned.
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