Single sign on solution in GAE and other Google services

The Google App Engine is launched for a while and many creative works have been developed. Since Google App Engine is provide by Google itself so it should be easy to process single sign on by using Google account. It's true for partial: according to the official document of GAE, developers could use User API to integrate with Google account easily.

However, if your application would like to communicate with other Google web services such as Google mail, Picasa Web, Google calendar ... etc, and tend to provide a single sign on interface to avoid too many authentication process for users then the solution of User API is totally not enough.

To achieve this goal, we need to use another Google API called "Account Authentication API". It's pretty much a Google version of Yahoo! BBAuth. The operation process to both solutions is very simular but Google's implementation provides better administrative interface.