Take the API documentation of Ruby and Ruby on Rails always with you on your iPhone or iPod Touch. For free!

Ruby Helper allows you to browse or search for classes, modules and methods. Tapping a class or module shows you the description and the methods of this class or module. Each method is then displayed with its parameters, its description and its source code. In addition, the view Texts provides you with introductory texts about Ruby and Ruby on Rails as well as its license information.

All of the documentation is stored on your iPhone or iPod Touch, so no internet connection is needed.