GPGMail: GnuPG on Mac OS Mail
Via Mike:
I usually use Mozilla’s Thinderbird and the GnuPG Plugin Enigmail.
Mail on Mac OS is quite good but never had a GnuPG integration. If you need to use Mail and GnuPG, there is GPGMail, but keep in mind:
GPGMail is a complete hack, relying on Mail’s private internal API. Use it at your own risks!
Mac OS X 10.5 is required and it is released under BSD license (recently changed).