Privacy Badger

uBlock Origin works from lists — the community maintains an enormous roster of ad and tracker domains, and anything matching gets blocked. That approach is effective but has a structural weakness: it cannot recognize a new tracker that no list has caught yet.

Privacy Badger fills exactly that gap. Built by the EFF, one of the most significant digital rights organizations in the US, 3,804 stars, GPL-3.0, and working on an entirely different principle: it learns.

The mechanism: as you browse, it records the behavior of each third-party domain. When a domain turns up on three unrelated sites while reading something that identifies you — cookies, localStorage, canvas fingerprinting — it concludes this is a cross-site tracker and starts blocking it. The entire judgment happens inside your browser, and your browsing history is never sent anywhere.

That produces an interesting property: the result is personal to you. Trackers on the niche sites you frequent may never reach a public blocklist, but if they follow you across sites, Privacy Badger notices. It gets more accurate the longer you use it.

Practical details: each domain has three states (allow, block cookies only, block entirely), the toolbar icon shows how every third party on the current page is being handled, and you can adjust manually. When blocking breaks a site, slide the offending domain back to allow.

To be clear about:

• It is not an ad blocker. The EFF says this explicitly: it blocks tracking behavior, and an ad that does not track you is left alone. For ad removal you still want uBlock Origin.
• Use both together rather than choosing between them. uBlock covers the known majority via lists; Privacy Badger catches what the lists miss through behavior analysis.
• It blocks almost nothing at first and needs browsing time to build up judgments. That is by design, not a fault.