git clone 'git://github.com/pd/yard-mode.el.git'
Rudimentary support for fontifying YARD tags and directives in ruby comments.
M-x package-install yard-mode.
Add it to your ruby hook:
(add-hook 'ruby-mode-hook 'yard-mode)
If you would also like
eldoc support, so that the expected syntax for
the tag beneath your cursor is displayed in the minibuffer, add that
(add-hook 'ruby-mode-hook 'eldoc-mode)