Rudimentary support for fontifying YARD tags and directives in ruby comments.

Use MELPA: 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 hook too:

(add-hook 'ruby-mode-hook 'eldoc-mode)

1. Some reasonable means of providing completions.
2. Tidy up the defcustoms; most could be parsed from yard-tag-docstrings.