Quizlet is an easy-to use and comprehensive site, boasting over 6 million sets of flash cards on every topic imaginable as well as games and quizzes. Flashcard Machine and Flashcard Exchange also have huge flashcard libraries, with over 40 million cards and counting. All three of these sites have a mobile app so your little scholar can study on the go.
Finally, KitzKikz is not a specific flashcard site, more a repository of a programmer's projects, but one of those projects is a flashcard generator that can convert your text into QR code, a cutting-edge study aide that may tempt even the most jaded student. (Image 1 by Flickr member susivinh licensed for use under Creative Commons. Image 2 by Flaschcard apps)