Fingerprints

Fingerprints of humans are supposedly unique, and sometimes used to track down criminals or to authenticate access to systems.

Toeprints have similar sorts of patterns, but are less-used since shoes and socks usually get in the way of their being left in accessible places.

Links

 * Wikipedia article
 * FBI fingerprint guide
 * Fingerprints and Passwords: A Guide for Non-Security Experts
 * Why fingerprints make lousy authentication tokens
 * Chaos Computer Club clones fingerprints to unlock iPhone
 * Adermatoglyphia: The Genetic Disorder Of People Born Without Fingerprints