This plugin supports markdown format by kramdown.
Redmine officially supports markdown since 2.5, but it requires Redcarpet gem. The gem doesn't get along with jruby because of its dependency on C extension (see this comment in Gemfile).
Kramdown is a pure-Ruby markdown converter and it doesn't have such a restriction.
Tested on Redmine 3.2.1, but it would work on Redmine 2.x.
Clone or download this repository in plugins directory.
Run bundle install.
Restart redmine.
Go to [Administration] > [Settings] > [General] and set [Text formatting] to Kramdown.
This plugin is distributed under GPL v2 license. A part of source code originates from Redmine.