This is neat: When you log into a webmail account using Safari in OS X Mountain Lion, Safari will offer to save the login info. So far, so familiar. The new trick is that it will also offer to set up your Mac apps with the same login. Thus, you sign in to Gmail and Safari will ask if you want to use your Gmail account with Mail, Messages and Calendar.
Mountain Lion Automatically Sets Up Mail, Calendars And Messages When You Log Into Webmail