Firefox Add-On Blocks Browser Tracking

Ghostery is a Firefox add-on that allows users to see who is tracking them and block the offending trackers.  From the extension's description:

Ghostery is built and maintained for users that care about their online privacy, and is engineered with privacy as a primary goal. Ghostery use is anonymous. No registrations or sign-ups are required. The Ghostery plug-in does not place cookies into your browser. Neither the Ghostery application nor Evidon receives any data from Ghostery users unless the user opts-in to participate in Ghostrank. Ghostrank data itslef is anonymous, is NEVER used for advertising targeting purposes, and is only shared in aggregated, non-personal, statistical form.