1. Duo Security Two-Factor Authentication. https://goo.gl/wT3ur9
2. FIDO Universal 2nd Factor. https://www.yubico.com/
3. Google acquires slicklogin, the sound-based password alternative. https://goo.gl/V9J8rv
4. Google Authenticator Android app. https://goo.gl/Q4LU7k
5. Google Cloud Messaging. https://goo.gl/EFvXt9