How unique is your browser?
The Electronic Frontier Foundation have a neat tool out: the Panopticlick. Many folks don’t know this, but every time you visit a web page your web browser sends lots of information to the web server you’re talking to — stuff like what web browser you’re using, what sort of pages you can read, which plugins you have installed and so forth. This is necessary in order for the remote web server to answer you appropriately. But it can be used to identify you.
How? Imagine that your web browser is just describing you: it might say that you have brown hair, blue eyes, fair skin, a mole on your left cheek, a slight limp, prefer wearing plaid shirts, never wear a hat, have a birthmark on your left ankle and so forth. None of those data are unique: the world is full of brunettes, full of folks with blue eyes and so forth. But there’re not that many brown-haired, blue-eyed, left-cheek-moled folks out there — and still fewer have fair skin, and fewer still have a slight limp, and fewer still have birthmarks on their left ankles.
Why does this matter? Well, it matters in the same sense that fingerprints matter. Every time you touch something, you’re leaving fingerprints — and every time you visit a website you’re leaving a fingerprint. Pretty nifty, huh?