These plug-ins enable you to integrate Yubico OTP support into existing systems.
Third party plugins can be discovered on GitHub for example.
Yubico has declared end-of-life for the YubiKey Validation Server (YK-VAL) and YubiKey Key Storage Module (YK-KSM). These have been moved to YubicoLabs as a reference architecture. See article, YK-VAL, YK-KSM and YubiHSM 1 End-of-Life.