KC-135. USAF
The U.S. Air Force looks set to finally add winglets to a portion of its KC-135 Stratotanker aerial refueling fleet. The service says putting upturned devices on the tips of the wings on the over half-century-old aircraft will improve their fuel efficiency and reduce drag, saving up to $65 million each year in operating costs as a result.
Winglets are already in widespread use on commercial airliners for exactly these reasons.