Encipher.it encrypts sensitive data — like credit card info — so you can send an email and only the recipient can decypher it with the correct password.
Obviously, you try to keep your email as secure as possible, but if anyone gets in, they get access to all your messages — including some in which you might have stored other passwords or sensitive information. If you'd like to send something a bit more private to your friend, you can use the Encipher.it bookmarklet to protect it using secure PBKDF2 encryption. Just type your text in Gmail, Facebook or whatever service you're using, hit the Encipher.it bookmarklet, and enter the password you want to use to protect your message. Then, text that password to your friend (or send it to them by some means other than email). When they get your email, they can just hit the Encipher.it bookmarklet, type in the password, and view your secret message. It's extremely simple, and a good way to give certain messages just a little bit of extra lock-down. Hit the link to try it out.