Google Multiple account login
Till now only single Google account login was possible at any time in a Browser.If you had set the "remember the password" option and are lazy like me to enter the password every time you want to check your mail,you might be using Firefox,Chrome,Safari,and IE for all your different GMail accounts.I always found it cumbersome to open my different GMail account on different Browsers and use one different browser for each of my GMail account .
Google has now enabled the option of signing in to multiple Google accounts simultaneously from the same browser.This option can be enabled by going to the Google accounts page and enabling the option : "multiple sign-in". If you don't see the option, it should be soon available.
After changing the Multiple sign-in option to "on" after clicking on the "change" link, Google informs that this is an advanced feature and that it 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 quickly switch to a new account.
"If you use 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."
For more info refer this