Google Analytics is a service provided by Google to web developers to provide information about visitors. This helps web developers understand who is visiting their site, from where, and using which browser.
Some see this as an invasion of privacy, and there’s really no easy way to identify sites that use Google Analytics until you’ve already sent your data away to Google.
Google just released the Google Analytics Opt-out Browser Add-on (BETA) which lets visitors opt-out of sharing their data with Google.
(via Mike Dopp)