Google Multiple Account Sign-in
Signing in to multiple Google accounts simultaneously from the same browser is possible. When you go to the Google accounts page, (pull down the menu under your account name after loging in – it’s in the upper right corner of your browser window) you will see the option: “multiple sign-in”.
After clicking on the “change” link, Google informs you 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 under your email address at the top of the page, so you can quickly switch to a new account. Select “Switch account” from the drop-down menu.
“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.” (Google help center)
When you enable this feature, the most significant change is that you’ll see a new drop-down under 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.
Another change is that Google’s URLs include a different number for each account: http://mail.google.com/mail/u/0/, http://mail.google.com/mail/u/1/, etc.
Now you can read your messages from two or more Gmail accounts by opening Gmail in multiple tabs.
There may be 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.