Log on Multiple Google Accounts at the Same Time

1. Google is rolling out an update to the Manage Account page for all of its users this week, according to the Google Operating System (unofficial) blog. The new account page will let you log in and out of multiple accounts at the same time using a drop down menu at the top-right of the page.
2. Once logged in, you'll toggle between the following Google's URLs: http://mail.google.com/mail/u/0/, http://mail.google.com/mail/u/1/, etc.

note:

        There are some known issues related to multiple sign-in: this feature is not available on mobile devices, Google Calendar's gadget doesn't work properly in Gmail, you can no longer use offline Gmail and offline Google Calendar and the "note in Reader" bookmarklet only works for the default account.