Just stumbled across this open-source project...
https://code.google.com/p/end-to-end/ is a Chrome extension that helps you encrypt, decrypt, digital sign, and verify signed messages within the browser using OpenPGP.
Not yet in web store-- you currently have to compile it yourself under linux (build scripts are included). Also, because it is based on PGP you need the recipient's public key first before you can send them encrypted messages.