It seems not well known and somewhat controversial, but offers generous rates higher than everywhere else. For example, 4.8% from Expedia, 5.6% from Orbitz. Unlike every other cash back program they do not cap the amount for flights.
JetCash is effectively real cash because many items are available cheaper than even from Walmart, Amazon, or Costco. Presumably Jet was taking a loss on those transactions in the short term. It really has been my favorite cash back program ever.