Google is in the process of rolling out a cool feature by which you can sign into multiple Google accounts simultaneously within the same browser!
Having multiple Google accounts is quite common these days, so this is a real handy feature for many users. “Multiple account login” feature was in testing for several days now, but Google is now rolling it out for everyone.
In order to enable multiple accounts sign in, go to the Google Accounts settings page and look for “Multiple Sign-in” option under Personal Settings. If you don’t see that option, it will probably be enabled soon.
Click on the Change link and you will see a warning sign where Google informs that this is an advanced feature and that it will only work for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code.
Select the first option to switch on the feature. Then click on Save Changes button. Log out and log in back to enable multiple sign-in. You should now see a drop-down next to your email address at the top of the page which you can use to quickly switch the accounts!