It is a plug in for Firefox.
"At a time when your ISP is tracking your online activities, sites you visit are doing the same (even the one you do not visit are able to track you), Google is not to be left out in the game, and the NSA is tracking everybody else, it’s easy to be depressed.
What exactly don’t these guys know about you? Or what can’t they know about you? For most people, the shocking answer, is not much. You see, privacy died about 10 years ago. There are things you can do to regain some semblance of privacy in your life, but with the wide variety of ways used to track you, only the most paranoid and tech-savvy has the skills to put up a good fight.
But while most of us can’t do very much, we can at least have fun tracking who’s tracking us, as we surf the Internet. If you use Firefox Web browser, an extension called Collusion can help you see which websites are tracking you. Install it like you would install any other Firefox add-on"
http://www.linuxbsdos.com/2013/07/17/collusion-see-whos-tracking-you-in-real-time/
