The Google Authenticator package contains a plug-able authentication
module (PAM) which allows login using one-time pass-codes conforming to
the open standards developed by the Initiative for Open Authentication
(OATH) (which is unrelated to OAuth).
Pass-code generators are available (separately) for several mobile
platforms.
These implementations support the HMAC-Based One-time Password (HOTP)
algorithm specified in RFC 4226 and the Time-based One-time Password
(TOTP) algorithm currently in draft.
