SocialHistory.js, code enables you to detect which social bookmarking sites your visitors use. How does SocialHistory.js know? By using a cute information leak introduced by CSS. The browser colors visited links differently than non-visited links. All you have to do is load up a whole bunch of URLs for the most popular social bookmarking sites in an iframe and see which of those links are purple and which are blue.

Website: http://azarask.in/blog/post/socialhistoryjs/