Redcar is a self-hosting Ruby editor. It's written in Ruby, it's extensible in Ruby and its primary focus is editing Ruby code. It's currently in active development and it's getting quite mature.
Ruby has had a number of self-hosting editors in the past. However, none of them have been as complete and usable as Redcar. And while it may not seem important to be using an editor written in Ruby, think about this: Redcar is extensible in Ruby. If you need a new editor feature, you don't have to use Emacs LISP, Vim scripting, Java, or anything other than Ruby.