Google Authenticator generates 2-Step Verification codes on your phone.
Ranked in these QuestionsQuestion Ranking
Pro Simple and straightforward
All you do after installing is log in with your Google account, and then you can add almost any account (that supports 2-factor authentication of any kind) quickly.
Pro Works offline
The app requires an internet connection to set up, but once that's done, you can disconnect from wifi entirely and it will still work.
Pro Storage efficient
Uses less than 20 mbs.