Emacs interface for Ruby documentation search engine http://www.omniref.com/
When you want to look up a certain term in Ruby documentation, just run
M-x omniref
if you've selected a region of a buffer, a new page will open in your browser with the search term. Otherwise you'll be prompted for a term you wish to search for.
If you find yourself using omniref a lot, maybe you'd like to bind it to something like:
(define-key ruby-mode-map (kbd "C-c o") 'omniref)