Google begin rolling out "multiple-sign-in" feature that only work for Gmail, Google Calendar, Google Sites, Google Reader, Google Voice, App Engine and Google Code. "Now, when you visit Google accounts, you might see "multiple sign-in" option. When enabled, a drop-down is 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."
"If you use multiple sign-in, the first account you sign in become your default account. When you visit Google products that don't support multiple accounts, you'll automatically sign in to you default account. If you sign out of any Google product while signed in to any account, you'll be signed out of all your Google Accounts at once," explains Google help center.
There're some known issues related to multiple sign-in: this feature isn't available on mobile devices, Google Calendar's gadget doesn't work properly in Gmail, you can no longer use offline Gmail and offline Google Calendar and the "note in Reader" bookmarklet only works for default account.