gpg4usb
Nix für den Mac, aber trotzdem interessant:
To say it straight in only one sentence: gpg4usb is a very easy to use portable-application, which combines a simple text-editor with a GnuPG-frontend to write, encrypt and decrypt your text-messages. Gpg4usb should work on almost any computer you’re working on, should it be a Linux-machine or even one with a Microsoft-OS running.
Almost the only thing required is an available usb-port you are allowed to access. With this application you can write safe and encrypted messages anywhere you are: should it be an internet-cafe, at work or somewhere else on holiday… and you always have the encryption-keys available for usage!

