Google Authenticator
Google Authenticator is an application that implements TOTP or HOTP security tokens from RFC 6238 in mobile apps made by Google, sometimes branded "two-step verification". Authenticator provides a six- to eight-digit one-time password which users must provide in addition to their username and password to log in to Google services or other sites. The Authenticator can also generate codes for third-party applications, such as password managers or file hosting services. Previous versions of the software were open source but subsequent releases are proprietary.