app.rb

require "rubygems"
require "rack/contrib/accept_format" # sudo gem install rack-rack-contrib (http://github.com/rack/rack-contrib/tree/master)
require "sinatra"
require "yaml"
require "json"

use Rack::AcceptFormat

get "/foo.*" do
  @data = %w[foo bar baz]
  pass
end

get "/foo.html" do
  "<ul>" + @data.map { |n| "<li>#{n}</li>"}.join("\n") + "</ul>"
end

get "/foo.yaml" do
  @data.to_yaml
end

get "/foo.json" do
  @data.to_json
end

=begin

$ curl localhost:4567/foo
<ul><li>foo</li>
<li>bar</li>
<li>baz</li></ul>                                                                                                                                                                                        

$ curl localhost:4567/foo --header "Accept: application/json"
["foo","bar","baz"]

$ curl localhost:4567/foo --header "Accept: text/yaml"       
--- 
- foo
- bar
- baz

=end