Have you noticed that, after you visit a web page, links to that web page change color, usually a lighter shade of blue? That’s one of the earliest User Interface wins of the web, a feature that dates all the way back to the first version of HTML. How convenient to be able to tell, at a glance, which links you’ve already visited!
Aza Raskin recently pointed out how this could be useful for reducing the number of social bookmarking links you see on articles: just show the ones the user actually visits. (In fact, he rediscovered something Nial Kennedy mentioned a couple of years ago, and about which web security researchers have already published a handful of papers.) Yes, indeed. Useful. And in the case of Aza’s code, not evil because it benevolently chooses not to report the info back to the server.
But still… a little bit scary, no?