When you go to the Google accounts page, you might see a new option: "multiple sign-in".

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.

Google Multiple Sign-in, Now Available

Other options for using multiple accounts : Getting started - Accounts Help