
Running multiple accounts

Not all Google services have the ability to use this feature. Currently, instant switching is possible for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. But we must assume that the list will expand.
When multiple accounts are enabled, next to your login in the upper right corner you will see a drop-down list of all connected accounts and you can switch between them in two clicks.

Also very observant will notice that the URLs of your mailboxes will slightly change. Now they will look like mail.google.com/mail/u/0 , mail.google.com/mail/u/1 , etc.
UPD Direct link to the edit page for multiple account settings → https://www.google.com/accounts/MultipleSessions