This only works with GMail, Google Calendar, Google Reader, Google Code, Google Voice 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 – Use multiple Google Accounts in the same web browser radio button. Check the associated check boxes and click the Save button.
Now when you login to any of the supported Google products, 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.
NOTE: When you have multiple sign-in enabled, you can no longer use the offline Gmail and offline Calendar.