This RPG produced by Fantasy Flight Games takes the fairy tales of The Brothers Grimm and twists them, creating a dark and macabre setting. The players assume the roles of ordinary elementary school children (pre-teens) who are dropped into a realm where these 'fractured fairy tales' have formed the basis of an alternate reality. The children must work together to survive and maybe even find the mythical way back home. It is available as both a D20 supplement and a stand-alone system.
Official site: http://www.fantasyflightgames.com/grimm/
Wikipedia article: http://en.wikipedia.org/wiki/Grimm_(role-playing_game)