Ri-viewer is a project mainly written in Vim Script, it's free.
ri-viewer.vim
This is a mirror of http://www.vim.org/scripts/script.php?script_id=3621
ri-viewer.vim looks up ruby document directories with a keyword such like class name or method.
You can retrieve details of class Object with bellow command:
:Ri Object
and a method detail
:Ri Object#help :Ri Object.help
If you want to look up documents included in specified dirs such like rubygems, home, try to bellow sub command:
:RiCore :RiGem :RiHome
RiCore retrieves ri documents included from /rubylib/system and /rubylib/site. RiGem retrieves from rubygems and RiHome does it from your home dir(~/.ri).
You can also get all classes and modules list:
:RiList
Also documents directories:
:RiDirs