Usage:

# as rails plugin
script/plugin install git://github.com/brynary/rack-bug.git

# as system-wide gem dependency
sudo gem install perfectline-rack-bug

# as rails project specific gem dependency
Rails::Initializer.run do |config|
   config.gem "perfectline-rack-bug", :lib => "rack-bug", :source => "http://gems.github.com"
end

rake gems:install

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

# add bookmarklet to browser
open http://RAILS_APP/__rack_bug__/bookmarklet.html

Thanks to:

Django debug toolbar
Rails footnotes
Rack's ShowException middleware
Oink
Rack::Cache