We have plenty of tools that claim your browsing the web privately. But how unique is your system? And are you leaking just enough data that makes you uniquely identifiable? How would you know if you were? How can you audit your privacy?
The technically savvy may be are aware their web browsers send "user agents" and additional optional data when making a web request. These optional data sets can be enough to create a unique fingerprint to identify and track you on the web.
http://amiunique.org examines your browser's/user-agent provided data to determine if you have a identifiable fingerprint.
The service also allows you to see how your fingerprint evolves as you try to obsifcate, spoof, or hide your data leaks.