RailSide

RailSide is OS X’s first native Ruby on Rails editor. Written in Cocoa, it fits well with the Mac OS desktop.

It is responsive, attractive, it is an IDE that feels like an editor.
Written from scratch with Ruby on Rails in mind, there is no need for a project file, just drop a Rails app folder on the dock, and your app’s source files will appear in the navigation view, organised by category with related models, views and controllers grouped together.
Once you’ve opened a project, the development server is booted, and the output will appear in the integrated console. Any errors that occur while you test the app with the built in browser are detected and it will direct you straight to the offending line of source code.