http://online.wsj.com/article/SB10001424052748704767804575654910216593180.html
http://krebsonsecurity.com/2010/12/what-you-should-know-about-history-sniffing/
What You Should Know About History Sniffing.
Web sites are using simple Javascript tricks to snoop into visitors’ Web browsing history.
This reconnaissance works because browsers display links to sites you’ve visited differently than ones you haven’t: By default, visited links are purple and unvisited links are blue. History-sniffing code running on a Web page simply checks to see if your browser displays links to specific URLs as purple or blue.
http://blogs.pcmag.com/securitywatch/2009/06/stealing_history_without_javas.php