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. 

WWW: http://tangent.org/?view=mod_trigger