Let's Encrypt is a new Certificate Authority that easily provides free certificates for everyone. It is sponsored by Mozilla, Akamai, EFF, Cisco, IdenTrust and Automattic. It is registered as a Public Benefit Corporation that is IRS-Tax Exempt and based in California, USA. It is a Leaf CA of IdenTrust and all certificates will essentially be signed by the IdenTrust Root.
They also propose a new standard, ACME, that will allow easy certificate issuance and installation as well as Domain Validation. They also have a client (Preview) which is, with the CA Software Open Source and available on GitHub.
The installation is as simple as
sudo aptitude install letsencrypt in all Debian-based distributions. However, in order to install the Preview, mentioned above, a tutorial is available at ReadTheDocs.