Researchers found that configuration information like data on the type of browser, operating system, plugins, and even fonts installed can be compiled by web sites to create a unique portrait of most visitors. Even without cookies, popular browsers such as Internet Explorer and Firefox give Web sites enough information to get a unique picture of their visitors about 94 percent of the time.

This means that most of us are actually a lot less anonymous than we believe we are. The data doesn't actually identify the Web user, but it creates a unique browser "fingerprint," that can be used to identify the user when he visits other Web sites.

Using JavaScript, Web sites are able to probe PCs and learn a lot. No single piece of data is enough to identify the visitor on its own, but when information like browser version, language, operating system, time zone details,what combination of plugins and fonts are installed are strung together a clearer picture emerges.

Using the private mode offered by some browsers can do nothing to stop this analysis. Private browsing may give you a certain level of protection from those who have access to your computer but it hasn't got to the point where it can provided protection against the companies that are profiling Web user.

Ref: http://www.computerworld.com/s/article/9176904/EFF_Forget_cookies_your_browser_has_fingerprints?source=CTWNLE_nlt_dailyam_2010-05-18