GNU Privacy Guard
The GNU Privacy Guard (GnuPG or GPG) is the standard for encrypting e-mail messages, instant messaging conversations and files on the Internet. It works by encrypting messages using asymmetric keypairs individually generated by GnuPG users. You generate a key-pair, give the public key to your friends and keep for yourself - and decrypt with - a private key which is protected by a passphrase.
Finding peoples keys
This little shell script will check a number of known nameservers for a persons public GnuPG/PGP key. Keys are identified by a number such as 0x79E48D63A1A23194.