Single Leg Romanian Deadlift. Very important to get the hamstring strong in a lengthened position. As this is the position the hamstring finds itself when controlling the ball in field hockey.
Lunge for strength exercise. This is such a common position in hockey, so it is wise to strengthen up the muscles that co-ordinate this movement in order to reduce risk of injury and to improve performance.
Lastly, a low back mobility exercise that ensures normal movement at the lumbar spine, since hockey players find themselves in a rigid low back posture a lot when playing, they are at a higher risk of developing a abnormal movement pattern at the low back and so increase risk of back pain. This video shows what normal movement is and how to practice it.