Let's Encrypt - Let’s Encrypt is a free, automated, and open certificate authority brought to you by the Internet Security Research Group (ISRG).
Google Authenticator - Google Authenticator is a multifactor app for mobile devices.
JSON Web Token - JSON Web Tokens are an open, industry standard RFC 7519 method for representing claims securely between two parties.
andOTP - andOTP is a two-factor authentication App for Android 4.4+
Amazon Cognito - Amazon Cognito lets you add user sign-up, sign-in, and access control to your web and mobile apps quickly and easily. It scales to millions of users and supports sign-in with social identity providers and enterprise identity providers via SAML 2.0.
Microsoft Authenticator - One app to quickly and securely verify your identity online, for all of your accounts.