Remote physical device fingerprinting - CAIDA : OUTREACH : papers : 2005 : fingerprinting
I don't know if I should classify this as really cool or somewhat disturbing. The author of this paper came up with a few different ways to use a computer's clock as a fingerprint. A computer can be identified without its cooperation, even if it is behind a firewall that performs NAT.

I don't think that it would be too hard to modify a TCP/IP stack to counter this sort of probe. I suspect that this will be an option for Linux sometime in the next few months. It will probably take a couple of years or more before Microsoft considers this to be a problem with Windows.

There are some legitimate security concerns raised by this paper. Whether your hat is black or white, if you care about network security or Internet anonymity then this paper is important to you.