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.