Smarthistory focuses on art history, from cave paintings to Warhol. And while the site calls itself a textbook, it's not the text or even the illustrations that make it special. It's the growing library of videos that feature spirited, unscripted conversations among historians about notable works. You can start in ancient times and work your way forward or browse the collection by artist, theme or medium. In 2010, the site's proprietors made their engaging creation available in portable form, with an iPhone app called Rome: A First Look.