Aside from bands or dumbbells, manual resistance exercises (where a coach, teammate, or training partner provides the resistance) are another great way to increase strength, stability and neuromuscular coordination in the muscles of the shoulder.
One benefit of these exercises is that they emphasize eccentric muscle action which is required during the deceleration phase of the pitching arm following ball release. Another benefit is that the amount of resistance can be altered as necessary to suit the individual. A final benefit is that the one providing resistance can ensure the desired muscles are actually doing the work.
Oh, and you don't need any equipment!