OpenOTP is an implementation of the HOTP protocol using a ZeitControl Cardsystems ZC3.9 BasicCard and standalone balance reader, standalone Spyrus PAR2 (Personal Access Reader), or PCSC-Lite supported Smart Card reader.

WinAuth is a portable, open-source Authenticator for Windows that provides counter or time-based RFC 6238 authenticators and common implementations. Also on GitHub - winauth/winauth.

multiOTP open source is a GNU LGPL implementation of a strong two-factor authentication PHP class. multiOTP open source is OATH certified for HOTP/TOTP. Also on GitHub - multiOTP/multiotp.