Google recently began rolling out “multiple-sign-in” feature that will work with various Google products including Gmail, Calendar, Sites, Reader, Voice, App Engine and Google Code. Once this feature is enabled, you should be able to sign in to multiple Google accounts simultaneously from the same browser.
Once this feature is enabled for your account you can go to Google accounts page, and you will notice a new option: “multiple sign-in”. Once this is enabled you, a drop-down will be displayed next to your email address, so you can quickly switch to a new account. Another change is that Google’s URLs include a different number for each account: http://mail.google.com/mail/u/0/ etc.
Google Accounts help page also provides following information about this feature:
If you use multiple sign-in, the first account you sign in to will be your default account. If you visit other Google products that don’t support multiple accounts after you’ve signed in, you will automatically sign in to your default account for that product. If you sign out of any Google product while signed in to any account, you will be signed out of all your Google Accounts at once.
Warning: Enabling multiple sign-in will disable Offline products like Offline Gmail and Offline Calendar, as well as any browser bookmarks you’ve set to link to your accounts. If you use Offline Gmail, make sure to sync your offline mail before enabling multiple sign-in so you don’t lose any messages in your outbox. If you would like to continue using Offline Gmail, Offline Calendar, and browser bookmarks linked to your accounts, do not enable the multiple sign-in option. If you have already enabled multiple sign-in, you may disable it.