freeOTP is a soft token authenticator that can be used for two-factor authentication, and is available as a free Android app or iOS app. Two-factor authentication — requiring a user to have “something you have”, as well as “something you know” i.e. a password — is being supported on many more services these days. Popular ways to implement the second factor include a hardware token, a YubiKey, or a soft token application that is on your mobile phone (as freeOTP does).
FreeOTP can currently be used for services utilising the HTOP and TOTP one-time password protocols, and also supports adding a new soft token via scanning a QR code generated by the service you are setting up authentication for. You can also use freeOTP as an open source replacement for Google Authenticator when logging into Google services.