Proton Mail is a free, web-based encrypted mail service founded by CERN in 2013. It uses client-side encryption, and could be termed a "trust no-one" mail service, since the data stored on the servers is encrypted, and the user never loses control of the encryption key.
Obviously, since the client-side is served rather than being resident on the client computer, this has raised some questions as to the overall trustworthiness of the service, but Proton Mail have addressed this by Open Sourcing the code - under the very permissive MIT license.
The code itself runs on Node.js, and leverages such modern web frameworks as Angular.js. It can be acquired fromhttps://github.com/ProtonMail/WebClient as can the very scant install instructions. These instructions, however, don't address any set-up or install of dependencies, so here is a more complete instruction for Ubuntu 14.04 LTS, Trusty Tahr.
$ cd /var/www
$ git clone https://github.com/ProtonMail/WebClient.git protonmail
$ cd protonmail
$ sudo apt-get update
$ sudo apt-get install nodejs
$ sudo apt-get install npm
$ sudo ln -s /usr/bin/nodejs /usr/bin/node
$ sudo npm install -g grunt
$ sudo npm install -g grunt-cli
$ sudo npm install -g bower
$ sudo npm install
$ sudo bower install --allow-root
$ sudo grunt --prod
The service will be be available on port 8080.
All I have done so far is verify that the site loads. I need to look closer at the code to determine what persistence is used, how to register new users, how to actually hook into an SMTP back-end and so forth.