[mercora]

I do not know why not more services use it but i want to point out that the Google Authenticator app is just an implementation of [0]HOTP and [1]TOTP.

There is also a free implementation of the same feature set available called [2]FreeOTP.

[0] https://tools.ietf.org/html/rfc4226

[1] https://tools.ietf.org/html/rfc6238

[2] https://fedorahosted.org/freeotp/