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