Yubikeys are USB tokens that act like keyboards and generate one-time passwords. The tokens are produced and sold by Yubico

This is a server that checks the validity of those OTP tokens. There are servers written in Java and PHP, while this one is written in C

It implements the server side of the API as described on http://www.yubico.com/developers/api/ and can be used with any client that implements the same API.