RubyOnRails Plugin: Auto Tags

AutoTags, an open source
project, is a plugin for RubyOnRails applications to automatically
generate the relevant tags for the provided content. Its quite simple
to use and provide easy integration with ActiveRecord. The plugin is
very useful for social applications, community websites and all those
networking applications where there is a scope of user generate
content to be tagged. No more heck for the admin to sit and add the
tags to the content or waiting for the users to add the tags to their
content.

For installation and usage read here.