I have more than a passing interest in cryptography, ciphers and steganography (probably due to my fascination with all things spy as a child). You can perform some mind-boggling things with the software freely available on the Internet, which does make me wonder exactly what super-secret techniques are in use by certain government agencies.
Anyway, I finally have two-factor authentication (i.e. something you have and something you know – a thumbdrive and password for example) working on my MBP. My GnuPG keys are all stored on my USB drive and using symbolic links I can now sign and/or encrypt emails or individual documents both at home (using Apple Mail.app with GPGMail) and at work (using Thunderbird with Enigmail) with the corresponding subkey.