Authy - Best rated Two-Factor Authentication smartphone app for consumers, simplest 2fa Rest API for developers and a strong authentication platform for the enterprise.
Okta - Enterprise-grade identity management for all your apps, users & devices
Google Authenticator - Google Authenticator is a multifactor app for mobile devices.
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.
OneLogin - On-demand SSO, directory integration, user provisioning and more
privacyIDEA - privacyIDEA is an enterprise ready, open-source multi factor authentication system.