Using Javascript,
you can walk through all of the links on your page and grab their styles; comparing a link’s style to the style given to visited links in the CSS will tell you whether or not a site has been visited. A site will know whether or not have been to any of the sites on their list. Combine that with a little Ajax, a database and a cookie, and you can track where your return users have been over time.
Check Jeremiah Grossman’s blog. Via Techfoolery.
