Google conventionally did not allow using accounts associated with different Gmail IDs at a same time. We have already discussed in the past ways to switch quickly between Gmail accounts. Now there is an official way to link different Google accounts so that you can use Gmail, Google Reader, Google Voice and other services with just a click of mouse within the same browser.
Why Multiple Sign-in is such a good idea?
Suppose you have two Gmail addresses that you use for different purposes. Traditionally, you can use only one at a time. For viewing another inbox, you have to sign out from one Google account and then sign in into another. With multiple sign-in feature you can directly access second Gmail inbox without signing out from the first one.
How to Sign in to Multiple Google Accounts?
To use separate Gmail/Google accounts in same window, you will have to add an account on top of already logged in account. First, open any Google product (Google search, Gmail, Drive, Map) and click on your name or picture at top tight hand side. Select Add Account from the drop down.
Alternatively, you can go to https://accounts.google.com/AddSession and add/remove accounts.
After enabling multiple sign-in (linking different Google accounts), you can see a drop-down along with your current Gmail id at the top-right corner of browser, which will let you choose other Gmail without login-out from the first one.
Please note that, this simultaneous logins will not work on all Google services. The current supported products are:
Also, Multiple Sign-in is not available for offline Gmail and offline calendar.
PS: It is better to check your current Gmail address before sending a mail. Current Gmail address is at top-right corner in browser or at bottom in mobile client.