Buying textbooks is expensive. Starting from that thesis, Chegg has experienced explosive growth by offering semester-long book rentals. Chegg ships a title at the beginning of class, lets you use it through finals and provides a free shipping label to facilitate the book's return, all at a steep discount off the list price. That, plus plans to venture further into digital textbooks and a slick iPod app for price comparison, means students will have more money left over for, um, Vitaminwater. (See the Chegg mobile app.)