[Let's Encrypt][1] is a new certificate authority that provides free certificates for web server validation. It issues [domain-validated][2] (DV) certificates meaning that the certificate authority has proven that the requesting party has control over some DNS domain (more on that later). And the best thing: it's fully automated through an [API][4] and a [command-line client][3].
Free DV certificates seem to be the new trend nowadays with Symantec being the next player in the market [announcing][5] they're giving them away for free. Let's Encrypt issued their [first][7] certificate on September 14, 2015 and announced on March 8, 2016 that they were at one million after just three months in [public beta][8].