Here is an interesting article about how "play" is being redefined in schools: How Children's 'Play' is Being Sneakily Redefined. It seems as though people are forgetting that unstructured play helps children with their motor skills, communication skills, social skills, problem-solving skills, cognitive skills, emotional development...I could go on and on. Parents, let your kids play!
A great resource is the Your Child At Play series. This book series discusses the benefits of play, gives great ideas of how to play with your child, and reviews several different types of play.