You are not as anonymous as you think....

From the Panopticlick Project:

Traditionally, people assume they can prevent a website from identifying them by disabling cookies on their web browser. Unfortunately, this is not the whole story.
When you visit a website, you are allowing that site to access a lot of information about your computer's configuration. Combined, this information can create a kind of fingerprint — a signature that could be used to identify you and your computer. Some companies are already using technology to try to identify individual computers. But how effective would this kind of online tracking be?

 Turns out your not really that unique and tracking data you can't really prevent can identify your PC down to 1 in 286,777.... that's like 18.1 bits of entropy....geez

Check out the white paper.