Great article on Wired discussing Leonardo Da Vinci and cryptology. I’ve always admired Da Vinci, and I admire him even more now that I realize just how privacy-conscious he was. Da Vinci created a few forms of cryptology, including a manual method of public key encryption. This ensures that only the intended recipient can read the message. Anyone intercepting the message cannot read it.
While Da Vinci used a glass tube filled with vinegar and manual dials to input the password, we can use purely digital means to protect our private messages. I highly recommend the freeware version of PGP (Pretty Good Privacy). It takes a little bit of work to download and install, but once you have it working and get public keys from your friends, you can send email that only the recipient can read. Anyone (say, the government using the powers granted to them by the Patriot Act) who intercepts the message will only see garbage.