Browser Fingerprinting

Browser Fingerprinting is a technique used to identify users based on the wide variety of data available to websites from the browser. It can successfully track users without cookies or other standard tracking methods. This is basically akin to identifying a person by visual inspection. Just by talking to someone, it's possible to actively reveal no identifying information. However, if you came back and talked again, you could be remembered by how you look, even if you supply no name, or even if you supply different names and identifications.

Panopticlick, a project under the EFF, found that
83.6% of browsers were uniquely identifiable, with 5.3% more were unique to only two users. With Flash or Java installed, the number of uniquely identifiable users goes up to 94.2%, with a further 4.8% unique to only two users. This leaves 1.0% of users who are members of groups with a size 3 or greater, meaning only one in one hundred standard web users have any real hope of being anonymous, regardless of the care taken with proxies, personal information, and security.

For more, see:
Browser Fingerprinting - Technique to Identify Users without using Cookies
How Unique Is Your Web Browser?