My personal favorite is the "Hershiser" drill, which aims at getting the hips fully involved.
One complaint I have seen frequently on this website is the pitcher having difficulty because he's been throwing with just the arm and the shoulder, when what he needs is to get the whole body into the action! I learned long ago, from watching how the Yankees' Big Three pitching rotation did it, is that by driving off the lower half of the body, using the legs, the hips and the torso in one continuous (and, it seems, seamless) motion they all were generating more power behind their pitches---and taking a lot of pressure off the arm and the shoulder so that they were throwing harder---and faster---with less effort (even Ed Lopat, who was not a fireballer). I picked up on that, worked with it, and got results. So when it comes to drills and exercises for mechanics, the "Hershiser" is one of the best. 8)