Implementation of the Google Identity Toolkit (openid like)
If you want to implement it in specific open source platform, please use Google Identity Toolkit Plugins. There is a plugin for opencart 1.4.x: http://www.opencart.com/index.php?route=extension/extension/info&amp;extension_id=3302

It only asks you for 3 installation steps without any coding work to make your dream become true! =)

Drupal plugin for it will be alive soon.