GPGMail is a plug-in for Mail.app, providing a front-end to GPG for some operations. GPGMail extends Apple’s Mail/MailViewer application and allows you to read and send PGP authenticated and/or encrypted messages. You can use GPGMail for plain and MIME messages.
Pros: Seamlessly and effortlessly integrates PGP encryption into Mail.app by using MacGPG.
Cons: It can take a while to learn how to effectively use GPGMail.
Note: Featured in Secure Your Email.