I use YubiKey which is an easy to use two-factor authentication, and Yubico have created a Salesforce app! It utilises the new Login Flows released by Salesforce in Winter ’15 so you can authenticate to Salesforce using a YubiKey!
From Goodreads
Your Baby Week By Week: The ultimate guide to caring for your new baby
How to Bake
Saving Bletchley Park: How #socialmedia saved the home of the WWII codebreakers