Google is rolling out a new feature that lets you sign into multiple Google accounts at once. This is a pretty great feature, and one that will save many people a lot of time.
As far as multiple login goes right now, you can currently be signed into only two separate accounts at once -- one Google Account, and one Google Apps account. This lets you be logged into your personal stuff, and your work stuff at the same time.
Until now though, there was no way for you to be signed into multiple Google Accounts -- say, three Gmail accounts. People maintain 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 choose which account you want to look at. I suspect it will be something like Google Analytics, where you can choose which account you want to view.