mod_trigger gives you hooks into each Apache request to launch triggers if certain actions occur. You specify the actions through directives that mod_trigger adds to the Web server.
This allows you to, for example, have a script email you when someone is visiting a certain page, let you know when someone from a certain domain is looking through your Web site, etc.
This will allow you to fire off scripts/CGI/servlets when these (and a few other dozen) events occur. No modifications are required to the content of your site and users of your server never need to know that a trigger has been put in place.