YARD is a documentation generation tool for the Ruby programming language. It enables the user to generate consistent, usable documentation that can be exported to a number of formats very easily, and also supports extending for custom Ruby constructs such as custom class level definitions. Above is a highlight of the some of YARD's notable features.
And of course YARD comes with much more functionality, including the ability to serve documentation for gems, the ability to group methods into logical sections, and much more. You can read about all of the new features in YARD 0.6 in the What's New document.
To install YARD with RubyGems:
$ gem install yard
And see all the commands:
$ yard --help
YARD is used in a wide variety of projects including personal ones that we don't know about, but here are some projects that we know do:
YARD also runs a live documentation server for the community, hosting docs for all RubyGems and Github projects. The project is at RubyDoc.info (formerly rdoc.info)