The MacGPG project provides GNU Privacy Guard (GPG) for MacOS X, along with a reference to quite some useful tools which help integrating it nicely with other applications.
Actually, I switched to the MacPorts gnupg package recently, because I needed IDEA support. To compile it:
```sh $ sudo port install gnupg +idea ```In case you don't need IDEA, the MacGPG package works just fine.
Either way, if you install GPG make sure to have a look at the Links section. Especially GPG Keychain Access, GPGMail and ABKey are must-haves.