I went to a great talk at linuxfest with a mozilla developer talking about ways you could prevent tracking, and some of the means were mind blowing. For example, they can process an audio clip using your audio hardware without playing it to you, and then based on the artifacts introduced, they can figure out if you are using the same sound card as a previous sample they collected. The firefox property that allows this is 'dom.webaudio.enabled', and it is not related to playback.
That's insane. http://webkay.robinlinus.com/ has some more interesting methods. One of my favourites is querying every IP address in your local network and seeing which ones are up to generate a profile on you.
