is a free, automated, and open certificate authority (CA), run for the public’s benefit.
Let’s Encrypt is a service provided by the Internet Security Research Group (ISRG). site 
Related: Caddy
talks
Let's Encrypt - a talk by Peter Eckersley . This video is licensed CC-by-3.0 , courtesy of CCC .
This talk will explain how Let's Encrypt works, and show you how to use it to automate HTTPS on a range of different Web, mail and other server types. It will cover obtaining new certificates, renewing them automatically, and enabling various advanced TLS security features. We also discuss the roadmap to a Web that is HTTPS by default.
Let's Encrypt - a talk by Seth Schoen . This video is licensed CC-by-3.0 , courtesy of CCC .
As we've called for widespread use of HTTPS, the cost and complexity of the certificate system has been an obstacle. See Can Security be Beautiful