RubyMotion lets devs build native iOS apps using Ruby instead of Objective-C. You can choose your own editor (Vim, Sublime, Emacs, etc.) to write code for the RubyMotion compiler. No need to use Xcode.
Since you are not required to use Xcode, some devs may ask: Can you still use Interface Builder? Yes. This brief article, complete with code samples, shows how: http://rayhightower.com/blog/2012/10/18/using-rubymotion-with-interface-builder/