Privacy Badger is a browser extension that automatically blocks hidden third-party trackers that would otherwise follow you around the web and spy on your browsing habits. Privacy Badger now has approximately 900,000 daily users and counting.

Third-party tracking—that is, when advertisers and websites track your browsing activity across the web without your knowledge, control, or consent—is an alarmingly widespread practice in online advertising. Privacy Badger spots and then blocks third-party domains that seem to be tracking your browsing habits (e.g. by setting cookies that could be used for tracking, or by fingerprinting your browser). If the same third-party domain appears to be tracking you on three or more different websites, Privacy Badger will conclude that the third party domain is a tracker and block future connections to it.

Privacy Badger always tells how many third-party domains it has detected and whether or not they seem to be trackers. Further, users have control over how Privacy Badger treats these domains, with options to block a domain entirely, block just cookies, or allow a domain.

Source: The New and Improved Privacy Badger 2.0 Is Here | Electronic Frontier Foundation