Radiant is a free-software content management system written in Ruby by John W. Long as a Ruby on Rails web application.
Radiant is limited to basic functionality for a CMS. The intended users are small groups or teams and thus the software leaves much room for extensions. All the content is stored inside a database. It's possible to use MySQL, PostgreSQL or SQLite. Radiant depends, like every Ruby on Rails application, on the installed adapters for the database.
It's distributed via a separate download or installable with RubyGems.