This may not be the ideal solution. This just manages request.referers in a session variable, then looping over each unique path prints a link to the screen. It's more of a "history" than a hierarchy for resources in an application. It is however, pretty straight forward to implement.

First create a before_filter in your application controller.Call it on every action. In the definition we'll parse the request.referer variable with the URI::parse method so that we're only dealing with the relative, not absolute, resource. We'll make sure that we're only storing unique paths and if the user arrives at an index action, we set the session[:breadcrumbs] variable to nil. This indicates that they are at the top level.

#app/controllers/application_contoller.rb
#...
before_filter :setup_breadcrumb_navigation
protected
def setup_breadcrumb_navigation
    if params[:action] == "index"
      session[:breadcrumbs] = nil
    else
      url = URI::parse request.referer
      if session[:breadcrumbs].nil?
        session[:breadcrumbs] = url.path.strip
      else
        session[:breadcrumbs] = session[:breadcrumbs] + ", "+url.path if session[:breadcrumb]
      end
      session[:breadcrumbs] = session[:breadcrumbs].split(", ").uniq.join(",")
    end
  end
end

The helper function just splits apart the string stored in the session[:breadcrumbs] variable, looping over the relative paths. We link to them, but first replace the ugly "/" and preface the display with some form of delimiter. I chose to use the "::" which looks nice to me.

#app/helpers/application_helper.rb
  def breadcrumbs
    content_tag :span, :id=>"breadcrumbs" do
      if not session[:breadcrumbs].blank?
        session[:breadcrumbs].split(",").uniq.map{ |breadcrumb|
            link_to(" :: "+breadcrumb.to_s.gsub("/"," ")+"",breadcrumb)
        }
      end
    end
  end

This is definitely a hack. You won't necessarily find a hierarchical relationship between links in the breadcrumb navigation. However, it will display the users history letting them quickly access a resource they just visited a page or two before. It's perhaps 80% of the functionality for only 5% of the work.

Oh yeah, putting this in a view

# app/views/layouts/application.html.erb
<%= breadcrumbs %>