From http://gnupg.org/, the homepage for The Gunu Privacy Guard:

• “GnuPG is the GNU project's complete and free implementation of the OpenPGP standard as defined by RFC2440 . GnuPG allows to encrypt and sign your data and communication, features a versatile key managment system as well as access modules for all kind of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications.”

Seahorse - Encrypted Key Manager

OpenPGP on Thunderbird: Uses Enigmail

OpenPGP on Evolution

There is also Mutt, Kmail, what else?

A list of GPG Keyservers

Audio tutorial on GPG at Linuxreality is here http://www.linuxreality.com/podcast/episode-47-openpgp/

Gnu Privacy Guard mini HOWTO in English, Dutch, Spanish, and German