Google began rolling out a new feature today that allowed users to login into multiple Google accounts at the same time from the same browser. Previously users had to resort to private browsing mode or use a different browser. Chrome users could use Google Account Switcher while Firefox users had the option of multiple accounts sign-in through Multifox and CookieSwap which is still useful for other services.
You can enable multiple account sign-in in your Google account by going to the multiple sign-in settings page. If you don’t see it, it will be enabled soon. As always, Google is rolling out the changes in stages.
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 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.
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.
[via Google OS]