On August 3rd, Google begin rolling out "multiple-sign-in" feature that only work for Gmail. Ay that time, the features was limited to 3 Google accounts. Later, the feature extended to other Google services such as: Google Calendar, Google Reader, Google Docs, Google Sites and Google Code, in addition to existing Gmail.
A year later, Google now allow you to sign into 10 accounts at the same time and use services like Gmail, Google Calendar or Google Reader.
After signing into 10 accounts, if you try to login another account, Google throws the following error message:
"You are already signed in to the maximum number of accounts. If you want to use another account you must sign out of all Google Accounts then sign in to the account you want."
During the extension of the feature, Google said that it will be added to other Google services when this feature goes public. However, still Google's products AdSense, AdWords, Blogger, App Engine etc. has not been enabled to use "multiple-sing-in".