Exception Notification Gem and Rails 3

All previous answers are outdated, you can now simply add this to your gemfile:

gem 'exception_notification', :require => 'exception_notifier'

And edit your production.rb config file as indicated in the readme:

config.middleware.use ExceptionNotifier,
  :email_prefix => "[Exception] ",
  :sender_address => %{"Exception Notifier" <support@example.com>},
  :exception_recipients => %w{you@me.com}