Google Analytics Opt-Out Extension keeps your data out of Google's tracking service

Google Analytics Opt-Out Extension Keeps Your Data Out of Google's Tracking Service
Firefox/Chrome/Internet Explorer: With a browser extension, users can
control their privacy and stop their data from being sent to Google
Analytics, a Google service which monitors and tracks traffic and
visitor data for web sites.
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Google Analytics
Opt-out Browser Add-on

Opt Out of Google targeted ads

Privacy: Opt Out of Google's Targeted Ads

There are some links in this article to different ways to opt out of the new Google ad-targeting system.