I recommend anyone interested in learning about online tracking to install Mozilla Collusion http://www.mozilla.org/en-US/collusion/ After seeing the amount of seemingly disparate information certain organizations can track (Google/Doubeclick, Discuss, Facebook, Twitter, etc) consider installing Ghostery http://www.ghostery.com/

Collusion takes seconds to install and you can forget about it for months while it builds an interactive web of tracking.

Ghostery protects your privacy by blocking a large list of web spies.