According to Gmail Blog, users will now be able to sign in with two additional accounts at the same time, and easily toggle back and forth between them easily. Some services like Picasa and Blogger does not support this feature yet (they will be logged in with your firstly logged in google account). Google calls the feature Multiple Sign-In.
Activation Process
You can visit google.com/accounts and click the link next to "Multiple sign-in." After you sign into your first account, you can sign in with up to two additional accounts from the new accounts menu in the upper right hand corner of Gmail, then easily toggle back and forth between them. You can even open multiple Gmail tabs - one for each of your accounts.
Important Points
Not all Google services support multiple account sign-in yet. For the services that don't support it (like Blogger and Picasa Web Albums), you'll be defaulted to the first account you signed in with during that browser session. So if you click a link from Gmail to Blogger, for example, you'll be logged into Blogger with the first account you signed in with, even if you clicked the link to Blogger from your second Gmail account.
Multiple account sign-in only works on desktop browsers for now, so if you use Gmail on your phone's browser you won't see this option yet.
Suggested reading
Gmail Blog
Google help center
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