To enable this feature go to google.com/accounts and click Edit beside the option Multiple sign-in.

You will be directed to a page where you will have to click on the check boxes that explain the rules of using multiple accounts. It also show you what all Google products support this feature. Click on check box and save it. Now once you sign into your first account you can sign into the others by clicking the arrow besides your id at the top right corner. You can either toggle between different accounts or login to a new google account.
This feature is only for the advanced users. The services in which you can use thisfeature are Calendar, Reader, Code , Sites and Gmail. For services like Picasa Web Album and Blogger don’t support this feature, you’ll be directed to the first account you signed in with during the same session.
