The Exception Notification Plugin provides a very handy way to notify you of Errors that happen within your Rails app.
The plugin README is located here:
http://dev.rubyonrails.org/browser/plugins/exception_notification/README
Additional Reading:
Global Error Handling with Exception Notification
A very basic setup tutorial is located here:
http://soakedandsoaped.com/articles/2006/08/31/exception-notifier-ruby-on-rails-plugin