Let's Encrypt
“Let’s Encrypt is a non-profit certificate authority run by Internet Security Research Group (ISRG) that provides X.509 certificates for Transport Layer Security (TLS) encryption at no charge. It is the world’s largest certificate authority, used by more than 400 million websites, with the goal of all websites being secure and using HTTPS. The Internet Security Research Group (ISRG), the provider of the service, is a public benefit organization.”