Google is rolling out a feature that was mentioned in previously: signing in to multiple Google accounts simultaneously from the same browser.

When you go to the Google accounts page, you might see a new option: "multiple sign-in". If you don't see the new feature, it will probably be enabled soon

After clicking on the "change" link, Google informs that this is an advanced feature and that it will only work for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. When multiple sign-in is enabled, a drop-down is displayed next to your email address at the top of the page, so you can quickly switch to a new account.

"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." (Google help center)