Ugly helper

helper.rb

    def filter_dropdown(select_options, options)
      current_text = select_options.find { |text, filter| filter == options[:current] }.try(:first) || select_options.first.first
      image = '<img height="6" width="8" src="/img/v/transparent_r1.gif" class="icn icn-ext-ctr-arrow-down">'
      link  = content_tag :strong do
        link_to("#{current_text} #{image}".html_safe, "#", :"data-simplelayer" => "##{options[:key]}")
      end

      uls = content_tag :ul, :id => options[:key], :class => "dropdown", :style => "display:none" do
        select_options.map do |text,key|
          klass = (options[:current] == key ? 'selected' : nil)
          content_tag :li, link_to(text, params.merge(options[:key] => key), :class => klass)
        end.join.html_safe
      end

      <<-EOS.html_safe
        #{options[:label]}
        #{link}
        #{uls}
      EOS
    end

Yes, that looks cluttered ;-)

Separating your 20 line method into shorter helper methods makes the code easier to read already (imho).
That might be just a start, but maybe you can also come up with more generic helper methods that can be used on more pages.

def filter_dropdown(select_options, options)
  current_text = select_options.find { |text, filter| filter == options[:current] }.try(:first) || select_options.first.first

  <<-EOS.html_safe
    #{options[:label]}
    #{link_xy(current_text, image_xy, key)}
    #{uls_xy(select_options, options)}
  EOS
end

def image_xy
  '<img height="6" width="8" src="/img/v/transparent_r1.gif" class="icn icn-ext-ctr-arrow-down">'
end

def link_xy(text, image, key)
  content_tag :strong do
    link_to("#{text} #{image}".html_safe, "#", :"data-simplelayer" => "##{key}")
  end
end

def uls_xy(select_options, options)
  content_tag :ul, :id => options[:key], :class => "dropdown", :style => "display:none" do
    select_options.map do |text, key|
      klass = (options[:current] == key ? 'selected' : nil)
      content_tag :li, link_to(text, params.merge(options[:key] => key), :class => klass)
    end.join.html_safe
  end
end

Fix: in line 6 it should not be key but options[:key]