Recently Elvis Andrus of the Texas Rangers had a 39-game hitting streak against the Cleveland Indians ended by an 0 for 4 night. I don't think a hitting streak against a single opponent is quite as impressive as a standard, "every day, no matter the opponent" hitting streak, but it's still pretty cool, made even cooler in Andrus' case, because the streak dated all the way back to his MLB debut on April 6, 2009, when he went 1 for 4 against the Indians. In more than five years to begin his career, Andrus had never played against Cleveland without collecting a hit.
His streak made me wonder about the longest single-opponent hitting streaks in baseball history.