Google is rolling out a new trait that lets you sign into multiple Google accounts at once. This is a pretty vast feature, and one that will save many people a lot of time.
As far as multiple login goes right now, you can at present be signed into only two separate accounts at once — one Google Account, and one Google Apps account. This lets you be logged into your own stuff, and your work stuff at the same time.
Until now though, there was no way for you to be signed into numerous Google Accounts — say, three Gmail accounts. People preserve multiple accounts for various reasons, now switching between them is a whole lot easier.
In supported applications, like Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code, there will be a dropdown that lets you decide which account you want to look at. I believe it will be something like Google Analytics, where you can choose which account you want to view.
