The YubiKey Validation Server (YK-VAL) is a server that validates Yubikey OTPs. It is written in PHP, for use with web servers such as Apache.
The server implements the Yubico API protocol as defined in:
http://yubico.com/developers/api/
This server talks to another service for decrypting the OTPs, to avoid storing any AES keys within the validation server. One implementation of this service is YKKSM:
https://github.com/Yubico/yubikey-ksm
Another implementation of a KSM using the YubiHSM hardware is Python-PyHSM:
https://github.com/Yubico/python-pyhsm
Note that version 1.x is a minimal centralized server. Version 2.x is a replicated system that uses multiple machines.