Panopticlick Determines How Unique and Trackable Your Browser Is, Even with Cookies Turned Off [Privacy]
Webapp Panopticlick examines how unique and trackable your browser fingerprint is even if you've disabled cookies in your browser.
Developed by the Electronic Frontier Foundation, the webapp uses information your browser shares with web sites by default—like installed plug-ins, system fonts, screen resolution, and so on—to create a fingerprint and test how difficult it is to track your browser across the internet. My browser's fingerprint, for example, was unique among the 1.6 million browsers already tested by the site.
For more information—including ways you can defend against this kind of fingerprinting—check out this PDF.
Panopticlick | EFF