Who has and uses multiple Gmail accounts during the day? I do, which is why I find this feature awesome – I can now log in to multiple Gmail accounts in one browser window. Once you turn this feature on, you’ll be able to seamlessly switch accounts using a drop down at the top right of the screen. No more having to open new browsers just to log in to Gmail… I regularly switch between Chrome, Firefox, and Safari solely for this purpose.
Instructions: to get started, go to www.google.com/accounts. Then click on the “edit” link next to Multiple sign-in.
This service currently only extends to Calendar, Code, Gmail, Reader, and Sites.
For more info, see the Official Gmail Blog.