Any text editor that supports Ruby syntax highlighting will do. This is available in most text editors aside from ones like Notepad. If you’ve already got a favorite text editor then chances are that it supports Ruby on Rails.

If you’re looking for something new or different, here is a list of the most popular Rails editors:

• Textmate – This free editor is popular with Mac users.
• Sublime Text – Similar to Textmate, this editor is available for Windows, Mac and Linux
• Atom – Similar to Sublime and produced by some Github folk, this editor is available for Windows, Mac and Linux
• Vim – When used along with rails.vim this is a powerful editor. However the learning curve is much steeper than most editors.
• Emacs – A popular vim alternative, Emacs has comprehensive Rails support
• E Text Editor – Another Textmate like editor, this one is popular amongst Windows users

If you prefer IDEs over text editors you should check out Which IDE should I use with Ruby on Rails?

http://viarails.net/q/Which-text-editor-should-I-use-with-Ruby-on-Rails