Google Authenticator - Google Authenticator is a multifactor app for mobile devices.
Okta - Enterprise-grade identity management for all your apps, users & devices
Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.
Duo Security - Duo Security provides cloud-based two-factor authentication. Duo’s technology can be deployed to protect users, data, and applications from breaches, credential theft, and account takeover.
Azure Multi-Factor Authentication - Azure Multi-Factor Authentication helps safeguard access to data and applications while meeting user demand for a simple sign-in process.
OneLogin - On-demand SSO, directory integration, user provisioning and more