This post was published more than a few years ago (on 2007-02-15) and may contain inaccurate technical information, outmoded thoughts, or cringe takes. Proceed at your own risk.
I guess technically you're not supposed to do this, but an interesting way to obfuscate a message would be to enter it in TinyURL and get back a short url that you could pass on, upon linking to which, the recipient would see your message in the browser address window (or the domain error message). Of course anyone who has access to the URL has access to the message, so it's not at all secure, but an interesting exercise.