Let's Encrypt is a certificate authority that launched on April 12, 2016. It provides free X.509 certificates for Transport Layer Security encryption. See my first article on Let's Encrypt here for more details: http://lesichkov.co.uk/article/20160528182844312058/lets-encrypt-first-impressions
Here is how to use on Ubuntu 16.04 LTS with an existing webserver (Apache, Nginx, etc).
apt-get update git clone https://github.com/letsencrypt/letsencrypt cd letsencrypt/ ./letsencrypt-auto
Follow the instructions on the screen to select which domain will be secured with a certificate.