GPGMail for Mac OS X Snow Leopard 10.6.

Extensive tutorial: http://www.securitygeneration.com/security/using-gpgmail-to-encrypt-email/

Download latest version of MacGPG2:

http://sourceforge.net/projects/macgpg2/files/

Download the latest version of GPGMail: http://www.gpgmail.org/download/index.html