:visited CSS technique to get the browsing history of a visitor

I didn't know about this until today, but the technique itself is so simple that it must have worked on every browser from the first introduction of the :visited CSS selector. Think about it: in your site you have a css style that colors the visited links with a specific color. Then you place in your page a list of links to other sites (maybe in a hidden container). Using Javascript, you inquire the computed style for each of the links. Taadaa! You now know the links in your list that the visitor has recently visited.

You can download a PHP demo of the technique from here. Ain't this neat?

Update: I since found another interesting article, showing how a site could (on certain browsers that are not Internet Explorer >:) )see if you are logged on to another. Here is the link.