EFF (Electronic Frontier Foundation) has put up a web site called Panopticlick
It collects every scrap of info from your browser that it can - a browser will divulge a lot in order to optimize its display of information - so a server can find your screen size, a list of fonts, and of course the operating system and browser versions. This is even without looking for cookies!
So a clever site could try to 'fingerprint' individuals by retrieving system details from the browser. It's not clear if anyone has developed a good strategy to hash the information into an easy-to-store indicator, but it probably isn't that hard to do.