Google has been recently testing multiple account sign-in, which would allow users to sign-in with multiple accounts to Gmail, Google Calendar, Google Reader, Google Sites and Google Code without having to sign-out of an account.
This feature has now been rolled out to users in phases and can be used to access multiple Google accounts. If you have not yet enable Google Multi Sign-in Feature, here are some simple steps for enabling the Google Multi Sign-in feature.
Step 1: Go to your Google Accounts Manage page (clicking on the link will take you there)
Step 2: Click on the Edit link next to Multiple sign-in listed under Personal Settings. If you do not see the link it will be rolled out to your account shortly.
Step 3: On the Edit page, select the radio button next to "On – Use multiple Google Accounts in the same web browser." and then make selections from the checkboxes below after you have read the details. After you have done that, just click the "Save" button.
That’s it, you will now be able to check emails, calendar, docs and more from multi-account using a drop down provided at the top of the screen. If you want to add another account, just click on the down arrow key and click on the link which says Sign in to another account and then provide your login credentials to it.
If you want to disable the multiple account sign-in in Google, just follow the same instructions. However, in Step 2 click the Radio button next to "Off – Use on Google Account at a time"
This is definitely one of the most useful features I have seen with Google or any other service because it takes out the hassle of managing and maintaining several different accounts.