Vortex generators. You have probably seen the vortex generators on other aircraft (see the Javelin below).
For STOL, these aircraft generally must operate at low speeds during takeoff and landing, so the flow speed over the wings tends to be low as well. Aircraft like the C-17 use vortex generators to create a higher-speed flow over the wings and control surfaces at these conditions to improve performance and controllability. In the case of the C-17, the vortex generators are located on the sides of the engine nacelles rather than on the wings but they still produce the same beneficial effects.