http://erikeldridge.wordpress.com/2010/02/21/simple-ruby-rack-router/

config.ru

require 'router'

use Rack::CommonLogger
use Rack::ShowExceptions
use Rack::Lint
use Rack::Static, :urls => ["/static"]

run Router.new

default.rb

require 'mustache'

class Default
  def self.call(env)
    markup = 'Hi {{name}}'
    data = {:name => 'Fred'}
    html = Mustache.render markup, data
    [200, {'Content-Type' => 'text/html'}, html]
  end
end

foo.rb

class Foo
  def self.call(env)
    [200, {'Content-Type' => 'text/plain'}, 'foo']
  end
end

router.rb

class Router

  def initialize(path='routes.yaml')
    @routes = YAML.load_file path
  end

  def call(env)

    # Lookup path in routes hash.
    path = env['PATH_INFO']
    until @routes.has_key? path do

      # Chop off nested directories until we have a hit or ...
      path = path.rpartition('/').first

      # ... path is empty, in which case, default to root
      path = '/' if path.empty?

    end

    # Load controller definition.
    require @routes[path]

    # Resolve controller's class name, eg '/foo : foo' --> "Foo".
    class_name = @routes[path].capitalize

    # Controller must be a valid Rack app.
    Kernel.const_get(class_name).call env

  end
end

routes.yaml

---
    /       : default
    /foo    : foo