<p>Some e-Marketing tools claim to choose which web page to display based on where you were before. That is, if you've been browsing truck sites and then go to Ford.com, your first page would be of the Ford Explorer.</p>

<p>I know you can get the immediate preceding page with HTTP_REFERRER, but how do you know where they were 6 sites ago? </p>

Javascript this should get you started:
http://www.dicabrio.com/javascript/steal-history.php
There are more nefarius means to:
http://ha.ckers.org/blog/20070228/steal-browser-history-without-javascript/
Edit:I wanted to add that although this works it is a sleazy marketing teqnique and an invasion of privacy.