Mailvelope is a browser extension for OpenPGP email encryption, available for Firefox and Chrome. It can be used with any webmail service, so you can continue using your own email address.
Mailvelope is free and
open source, in active development since 2012, and uses the well-vetted OpenPGP.js. It has been adopted by GMX and Posteo for their integrated end-to-end encryption.
As it's based on the OpenPGP standard, Mailvelope is interoperable with any OpenPGP-compliant email client.
Just released,
Mailvelope 1.4 adds file encryption and public key search.
Mailvelope basics - read!
Chrome extension, from the Chrome Web Store
Firefox extension, from mailvelope.com