The functionality, which is in the process of becoming publicly available today, can be enabled by going to the Google Accounts Management Dashboard and looking under Personal Settings. The "Multiple sign-in" setting, which is off by default, should appear below the "Email addresses" setting. To enable it, click on "Change." According to Google, if it's not there yet, you should see it soon . Important: whatever account you're signed into when you enable multiple sign-in will become your default account.
The feature, which Google considers advanced, works with Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. A drop-down displayed beside the email address on the page allows you to switch to a new account. Google says that the ability to use this with Google Docs is in the works.
The new feature means that if you sign out of one Google account, you are signing out of all Google accounts that you are currently logged into. When logged into multiple accounts, you can tell which one you're currently using by looking at the top of the page in Google product pages, or at the bottom of the page on other sites. Google's URLs will change with the use of the new feature in that each will contain a different number for each account.
You can sign into a maximum of three accounts at once. Known issues at this point include an inability to use multiple sign-in on mobile devices and the loss of the ability to use Offline Mail or Calendar.