Like everyone else said. . . . ass to floors (squats). Also, lying leg curls hit the lower buttocks to a certain degree.
Here are other exercises that will hit the area:
Wide Stance Leg Presses:
Put feet high on platform and let the quads travel towards the side of the chest.
Lunges:
These certainly target the buttocks. Of course, you will stress the hams and buttocks the further you lunge forwars.