Google is reported of "testing a new feature that allows you to sign in to "multiple Google accounts" in same browser. As of now, it only works for Gmail, Google Calendar, Google Reader, Google Docs, Google Sites and Google Code, but likely other Google services will be added when this feature goes public." If you use a service that doesn't support multiple sign-in, it'll default to first account that you signed in using your browser. Google suggests to use a different browser, but you can also use Chrome's incognito mode to sign in to an additional Google account.
Another limitation is that Gmail's offline mode is disabled if you turn on multiple sign-in. Multiple sign-in will let you open Gmail in multiple tabs, log in using different accounts and read the messages from all your accounts without opening another browser. Right now, you have to log out before logging in to a different account.