For a while, I've liked the idea of a client-side PGP plugin for Gmail. Essentially, you have a greasemonkey (or bookmarklet, whatever) script which performs the encode on send and save draft, and the decode on receive.
Of course, it kills Gmail's big feature, which is search. But for that, I figure you could take the wordlist from the email, hash each one individually, and then paste that at the bottom of the message. So your searches would still find matching messages, they'd just be a garbled mess to Google or any interceptor.
This doesn't seem like it'd be terribly complicated, but I don't think anyone's done it.
Of course, it kills Gmail's big feature, which is search. But for that, I figure you could take the wordlist from the email, hash each one individually, and then paste that at the bottom of the message. So your searches would still find matching messages, they'd just be a garbled mess to Google or any interceptor.
This doesn't seem like it'd be terribly complicated, but I don't think anyone's done it.