Guardian
Guardian is a CGI script which works closely with the Apache 1.1 and later webservers. When a visitor to your site attempts to view a document which does not exist, tries to access a restricted area and fails, or initiates a script error, the Guardian awakes. He will provide the wayward visitor with helpful tips to get him back on track and, more importantly, will fire off a priority email to the site administrator alerting him of the transgression.