デザイン作成時等、静的HTMLサーバをたてるときに。publicフォルダにリソース突っ込んでくれればOK.

config.ru

require 'sinatra'
helpers do
  def directory_index(path)
    ['index.html', 'index.htm' ].map{|i| "./public/#{path}/#{i}" }
  end
end

get '*/' do
  path = request.path_info
  p path
  directory_index(path).each do |index|    
    return File.open(index, File::RDONLY) if File.exists?(index)
  end
  raise Sinatra::NotFound.new
end

run Sinatra::Application