You may not know the name, but you know his impact. David Hume was one of the great philosophers in Western history. Interesting in his own right, he was a significant economist, being an early opponent of mercantilism, and early articulator of the quantity theory of money.
He had a significant impact on Adam Smith. He even helped develop the divide between normative and positive economics, having once noted "you cannot deduce ought from is." Value judgments cannot be made purely on the basis of facts. For more about Hume, try here and here.