When you go to the Google accounts page, you might see a new option: “multiple sign-in”. If you don’t see the new feature, it will probably be enabled soon.
And after click on the “change” link, Google will inform you that this is an advanced feature and that will only work for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. When multiple sign-in is enabled, a drop-down is displayed next to your email address at the top of the page, so you can do quickly switch to a new account.
“If you are using multiple sign-in, the first account you sign in to will be your default account. If you visit other Google products that don’t support multiple accounts after you’ve signed in, you will automatically sign in to your default account for that product. If you sign out of any Google product while signed in to any account, you will be signed out of all your Google Accounts at once.”
When you enable this feature, the most significant change is that you’ll see a new drop-down next to your email address in Gmail and other supported Google products. Click on the drop-down and you can sign in to a new Google Account without signing out from the previous account.
Sources : Google Operating System