How To Log-in with Different Gmail or Google Accounts in the Same Browser

Google conventionally does 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, first you will have to enable multiple sign-in from here [multiple log-in]
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.
According to Google Support, Google uses cookies
for sign-in process. Based on the cookie value, you can use only one account at a time. However, Multiple Sign-in allows you to use more than one account at a time in the same browser.
Please note that, this simultaneous logins will not work on all Google services. The current supported products are:
• Code
• Calendar
• Finance
• Gmail
• Reader
• Sites
• Voice
• Docs coming soon
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.