application_helper.rb

module ApplicationHelper
  
  def ul(opts={}, &block)
    ListBuilder.new(@template, block, opts)
  end
  
  def ul_nav(&block)
    ul(:class => 'nav', &block)
  end
end

example.html.erb

<% ul_nav do |ul| %>
  <%= ul.link_to "hello", '#' %>
  <%= ul.link_to "middle", '#' %>
  <%= ul.link_to "goodbye", '#' %>
<% end %>

<ul class="nav">
  <li class="first"><a href="#">hello</a></li>
  <li><a href="#">middle</a></li>
  <li class="last"><a href="#">goodbye</a></li>
</ul>

<% ul_nav do |ul| %>
  <% ul.li do %>
    <%= link_to "Hello", '#' %>, my child...
  <% end %>
  <% ul.li do %>
    <% 3.times do %>
      <%= link_to "middle!", '#' %>
    <% end %>
  <% end %>
  <%= ul.link_to "goodbye", '#' %>
<% end %>

<ul class="nav">
  <li class="first">
    <a href="#">Hello</a>, my child...
  </li>
  <li>
    <a href="#">middle!</a>
    <a href="#">middle!</a>
    <a href="#">middle!</a>
  </li>
  <li class="last"><a href="#">goodbye</a></li>
</ul>

list_builder.rb

class ListBuilder
  class List
    
    def initialize(template)
      @items = []
      @template = template
    end
    
    def li(&block)
      @items << ListItem.new(nil, &block)
      nil
    end
    
    def method_missing(method, *args, &block)
      @items << ListItem.new(method, *args, &block)
      nil
    end
    
    def generate(spacer)
      @items.each_with_index do |item, i|
        place = case i
        when 0:               :first
        when (@items.size-1): :last
        else
          nil
        end
        item.generate(@template, place.to_s)
        @template.concat(@template.content_tag('li', spacer || '|', :class => 'spacer')) unless place == :last
      end
    end
  end
  
  class ListItem
    
    def initialize(method, *args, &block)
      @method = method
      @args = args
      @block = block
    end
    
    def generate(template, place=nil)
      if @method.nil?
        template.concat("<li class=\"#{place}\">")
        template.instance_eval(&@block)
        template.concat('</li>')
      else
        template.concat(template.content_tag(
          'li',
          template.send(@method, *@args, &@block),
          :class => place
        ))
      end
    end
  end
  
  def initialize(template, block, opts={})
    template.concat("<ul class=\"#{opts[:class]}\">")
    list = List.new(template)
    block.call(list)
    list.generate(opts[:spacer])
    template.concat('</ul>')
  end
end