A new authenticator app has been built for Mac OS 8,9, X - TWOFAP (Two Factor Authentication Program).

This first version of TWOFAP was made using C++ and Qt; the source is available here.
The application has since been re-written in ANSI C (aka C89) using the Nuklear GUI library, in order to target more (and older) platforms, mostly for fun. The functionality of both versions is currently identical.

The Mac OS 8.1 - 9.0.x version requires CarbonLib 1.0.2 or higher.

Go to the developer's website.