Google has just introduced a new feature that allows users to grant other users limited access to their Gmail account, so that the delegated user can read, send, and delete messages on the owner’s behalf. This feature was originally available to Google Apps customers, which are usually business organization and people who have assistants who read and reply to emails on their behalf. This has been now rolled out to all Gmail users.
To grant access to another account, click the Settings link in the top right corner of Gmail. On the "Accounts" tab, you'll see a new section where you can "Grant access to your account." For example, below we've given firstname.lastname@example.org access to the email@example.com account.
The account you add will get a verification email with links to accept or deny access. Once the account accepts, a small down arrow will appear beside the email address at the top right corner of Gmail which can be used to toggle between accounts — in this case firstname.lastname@example.org and email@example.com.
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 firstname.lastname@example.org while signed in as email@example.com, it will appear as being sent by firstname.lastname@example.org on behalf of email@example.com.
[via Gmail blog]