Most common trackers
A curated educational catalog of the actors you see most often on the open web — who they are, which domains they use, and how they typically proceed from a page load to a profile or ad decision.
What this list is
This is an editorial shortlist for education, not a live census. Rankings of “most common” come from industry prevalence (ad tech, analytics, measurement), not from our website tracking audit directory. Names and domains change; treat examples as representative.
For how the mechanisms work (pixels, device graphs, audiences), start at the ad tracking hub.
How trackers typically proceed
Most third-party trackers follow a similar pipeline:
- Load — a script, pixel, or tag manager snippet runs on the publisher page.
- Identify — a first- or third-party cookie, storage key, or probabilistic ID is read or written.
- Event — a beacon reports page view, click, purchase, or scroll data to the tracker’s servers.
- Match / sync — cookie syncing or logged-in accounts link that ID to other sites, apps, or partners.
- Use — analytics dashboards, retargeting audiences, measurement panels, or ad bidding.
Deep dives: pixels & beacons, cross-device, audiences & brokers.
Actor catalog
Each entry lists typical domains and how that actor usually proceeds.
Google Ads / DoubleClick
Category: advertising · Domains: doubleclick.net, adservice.google.com
Publisher or advertiser tags load Google’s ad stack. Cookies and request parameters identify browsers across sites; bids and creatives are served via DoubleClick and related hosts. Logged-in Google accounts strengthen cross-site matching.
Google Analytics / Tag Manager
Category: analytics · Domains: google-analytics.com, googletagmanager.com
GTM injects tags; Analytics collects pageviews, events, and conversions into a property ID. Client IDs in cookies stitch sessions. Often co-deployed with Ads for attribution and remarketing audiences.
Meta Pixel
Category: social / advertising · Domains: facebook.net, connect.facebook.net
The Meta Pixel (and Conversions API) fires on partner sites for PageView, Purchase, and similar events. Matching uses cookies and hashed identifiers when available, feeding Meta ad targeting and measurement. See also social network tracking.
TikTok Pixel
Category: social / advertising · Domains: TikTok Events / pixel hosts
Similar to Meta: a site pixel or server events API reports conversions back to TikTok Ads. Used for optimization and retargeting of people who visited or purchased on advertiser sites.
Microsoft Clarity / Bing Ads
Category: analytics / advertising · Domains: clarity.ms, Bing ad hosts
Clarity records session replays, heatmaps, and interaction metrics for site owners. Bing Ads tags support search and display campaigns with Microsoft’s identity graph when users are signed in.
Amazon Advertising
Category: advertising · Domains: Amazon ad / attribution tags
Advertisers place Amazon pixels or tags to measure store visits and purchases. Amazon’s retail identity and advertising products link off-Amazon browsing to shopping behavior where matching is allowed.
Criteo
Category: retargeting · Domains: Criteo cookie / sync hosts
Classic retargeting: browse a product elsewhere, then see related ads on other sites. Relies on third-party cookies and ID syncs with publishers and exchanges. See audiences & data brokers.
Taboola / Outbrain
Category: content recommendation / advertising · Domains: taboola.com, outbrain.com
“Recommended for you” widgets load on publisher pages, set or read IDs, and report interest signals used for native ads across their networks.
Hotjar
Category: session analytics · Domains: hotjar.com
Loads a script that can record mouse moves, clicks, and page snapshots for UX analysis. Not primarily an ad exchange, but it collects rich behavioral data on the sites that install it.
Segment / Mixpanel / Amplitude
Category: product analytics · Domains: segment.io, segment.com, mixpanel.com, amplitude.com
SDKs and tags stream product events (sign-up, feature use) to analytics backends. Often first-party on the product domain, but can still identify returning users and feed marketing tools.
Comscore / ScorecardResearch
Category: measurement · Domains: scorecardresearch.com
Panel and census-style measurement tags help rate publishers and campaigns. Beacons report which sites and content were viewed for industry ratings and advertising currency.
What you can do
- Use a content blocker or privacy extension that blocks known tracker domains.
- Prefer browsers with strong tracking protection (Firefox, Brave, Tor, Mullvad Browser).
- Enable Global Privacy Control where supported.
- Run your privacy audit to see fingerprint exposure in this browser.
- Browse the website tracking audit directory for how sites load third parties in practice.
Check your browser exposure
Run your privacy auditFrequently asked questions
Is this list ranked by our Sites directory?
No. It is a curated educational list of widely deployed actors. The Sites directory aggregates opt-in audits and may show different third parties per domain.
If I block these domains, do sites break?
Often not, but some sites rely on tag managers or login widgets that share hosts with trackers. Blocking is a trade-off between privacy and functionality.
Are CDNs and consent managers “trackers”?
Not always. CDNs may serve functional assets; consent managers may store preferences. Classification depends on purpose — our extension’s Disconnect-backed list labels advertising, analytics, and social categories separately.
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