Now, you can visit google.com/accounts and click the link next to "Multiple sign-in." After you sign into your first account, you can sign in with up to two additional accounts from the new accounts menu in the upper right hand corner of Gmail, then easily toggle back and forth between them. You can even open multiple Gmail tabs — one for each of your accounts.
Please keep in mind that this is a feature for advanced users, and there are a couple things to watch out for:
1) Not all Google services support multiple account sign-in yet. For the services that don't support it (like Blogger and Picasa Web Albums), you'll be defaulted to the first account you signed in with during that browser session. So if you click a link from Gmail to Blogger, for example, you'll be logged into Blogger with the first account you signed in with, even if you clicked the link to Blogger from your second Gmail account.
2) We’re still working on making Gmail and Calendar work offline with multiple sign-in. If you rely on offline access, you probably don't want to enable this feature quite yet.
3) Multiple account sign-in only works on desktop browsers for now, so if you use Gmail on your phone's browser you won't see this option yet.
Since Google Apps customers can already sign in to their accounts at the same time as their personal Google Accounts, we won’t be adding this new feature to Google Apps until the new infrastructure is in place.
If you use more than one Google Account, we hope this makes you more efficient. If you have any questions, check out our help center.