Here’s an interesting way to check out your website visitor’s selective history. See it in action here. At the very least, it would be useful to check if they’ve been to your competitor’s website 
Jesse Ezell Blog : Stealing History
…it appears that basic technique involves setting the visited link color via CSS on a group of links to common sites, and then getting the computed values of the links and seeing which ones have the visited color.
(Thanks Neil)