[link|http://macgpg.sourceforge.net/|http://macgpg.sourceforge.net/]
Don't forget to install GPG Keychain Access and GNUPG.
You'll probably also want this:
[link|http://www.sente.ch/software/GPGMail/English.lproj/GPGMail.html|http://www.sente.ch/...proj/GPGMail.html]