Google Authenticator provides Two Factor Authentication and can issue codes for multiple accounts from the same mobile device. However, every Google Account needs a different secret key, which you will get by going through the setup process. If you set up 2-Step Verification using SMS text message or Voice call and also want to be able to generate codes using the Android, iPhone, or a Blackberry, you can use the Google Authenticator app to receive codes even if you don’t have an Internet connection or mobile service.
Authy is free to use for less than 100 authentications per month. After which it follows a pay-as-you-go model where you are charged per authentication.
2 Factor Authentication Services in US Authy