mcassoc is a free service that allows you to add Minecraft account association to your site or forums. By using skins to verify association, users don't need to provide account details to 3rd-party sites and once they've signed up once they simply need to enter their separate mcassoc credentials if they wish to associate on a future site.
Integration is simple and sample code is available in both PHP and Python.
To learn more and get your shared key, visit mcassoc.lukegb.com
This is not an official Google product.