If you have more than one Google account, going back and forth between the various Gmail, Calendar, and Reader sites for each account can be rather painful. By enabling Multiple Sign-in, you can easily access these different accounts in a simple manner. This only works with Gmail, Google Calendar, Google Reader, Google Code, and Google Sites.
1. Go to the Google Accounts page and sign into your default (primary) account.
2. Under Personal Settings, go to Multiple sign-in and click the Edit link.
3. Click the On radio button. Read the information beneath the radio button and check the associated check boxes.
4. Click the Save button.
Now when you login to Gmail (or one of the other supported Google applications), you will see a dropdown arrow next to your email address in the upper-right corner. Click the arrow and sign in with your additional accounts.
Once you have signed into each account, they will be added to the dropdown list. From now on, all you have to do to access the accounts is use the dropdown list and select the desired account. The one major downside to this option is that when you have multiple sign-in enabled, you can no longer use the offline Gmail and offline Calendar.