In Pictures: OS X Mountain Lion arrives - the top ...
Read the Web when offline
Apple introduced the Reading List capability more than a year ago to Safari, letting you save Web pages to be read later. Once read, they disappear from the Reading List. In OS X Mountain Lion, Safari now saves the HTML files so that you can read your saved Web pages when not connected to the Internet, such as on a plane or train. (iOS 6 will gain this feature as well.)
It's smart enough to save all pages in a multipage article, not just the page you were on when adding it to the Reading List.
In Pictures: OS X Mountain Lion arrives - the top 25 features