By tapping Diamond Valley you can sacrifice a creature in order to gain life equal to the sacrificed creature's toughness. Not only that, but it's effect is able to be used after blockers have been declared.
While it's not really a card that can necessarily win you a game, it's a very fun card for EDH decks that are built around life-gain mechanics. It is pricey, but it's a fun card to play. You can use it quite a number of times in a game, so it's worth a look if you're willing to sink two hundred bucks or more into acquiring a copy. This card is, understandably, on the Reserve List.