Windows 7: Browsers can leave a unique trail on the Web.

The Electronic Frontier Foundation has created an online tool that details the wealth of information a Web browser reveals, which can pose privacy concerns when used to profile users.

The EFF's Panopticlick tool takes just a few seconds to pluck out information that a Web browser divulges when visiting a Web site, such as a user's operating system, version numbers for plug-ins, system fonts and even screen size, color and depth.

Taken together, that information is a unique fingerprint for a particular PC, which could be used to repeatedly identify a particular visitor a Web site, the EFF said.

The EFF, which has campaigned against intrusive online advertising systems, warns that advertising companies are already using digital fingerprinting techniques, wrote Peter Eckersley, an EFF staff technologist, on the organization's blog.