Rails Plugin

 # install
 $ script/plugin install git://github.com/ddollar/rack-debug.git

 # config/environments/development.rb
 config.middleware.use "Rack::Debug"

Gem

 # install
 $ gem source add http://gems.github.com
 $ gem install ddollar-rack-debug

 # add a use line to your builder
 require 'rack/debug'
 Rack::Builder.new do
   use Rack::Debug
   run MyApp.new
 end

Add breakpoints to your code

 @user = User.find(params[:id])
 debugger
 render :show

Debugging:

 # run the rake task,
 $ rake debug
 Connected.

 # refresh a page in your browser, your app will break at debugger statements
 (rdb:1) p @user
 #<User id: 1, name: "David Dollar", email: "ddollar@gmail.com", created_at: "...", updated_at: "...">

Thanks to:

  Rack::Bug for a good example of Rack middleware in Rails
  Ben Scofield for making me want to build some Rack middleware