Gmail added "email delegation" feature that unitl now was only available to Google Apps accounts, and it's useful for people who want their assistants to have access to read or respond to mail on their behalf. To grant access to another account, click Settings link in the top right corner of Gmail. On "Accounts" tab, you'll see a new section where you can "Grant access to your account."
The account you add will get a verification email. Once the account accepts, a small down arrow will appear beside the email address at top right corner which can be used to toggle between accounts:
Each account will open in a different browser tab or window so you can view both accounts simultaneously, all while signed into your primary account. When you send a message from a account while signed into another, it'll appear as being sent by ….on behalf of …..
Signing out of any one of the accounts will sign you out of all the accounts you're currently viewing, and, of course, you can revoke access at any time.