Although backpropagation was rediscovered and popularized more than 30 years ago (Learning representations by back-propagating errors, D. E. Rumelhart, G. E. Hinton, and R. J. Williams, Nature, 323: 6088, pages 533–536, 1986), it still remains one of the most widely used algorithms to train artificial neural networks very efficiently. If you are interested in additional references regarding the history of backpropagation, Juergen Schmidhuber wrote a nice survey article, Who Invented Backpropagation?, which you can find online
