Browser fingerprinting is a technique to "mark" anonymous users using JS (or other things). To build an "identity" of sorts the browser is queried for a list of its plugins, the screen size and several other things, then hashes them. The idea is that these bits of information produce an unique "fingerprint" of sorts; the more elaborate the list of data points is, the more unique this fingerprint becomes. And you wouldn't even need to set a cookie to recognize this user when she visits again.
For more information on this topic consult Ars Technica or the EFF. There is a lot of potential for undesirable shenanigans, and I strictly oppose using this technique for marketing and ad-related tracking purposes.
Anyways, I needed a really simple fingerprinting library, so I wrote a quick and dirty jQuery plugin. This is by no means a complete and watertight implementation -- it is merely the scratch for a particular itch I was having. YMMV.
Documentation (docco-generated) can be found in the