I use med balls all the time. My favorite move is the ball slam because it hits so many muscle groups. WARNING****You need to use a non-bouncing med ball for this exercise. You basically raise the med ball over your head and slam it to the ground, then squat down to pick it up and repeat. It will give you a great cardio, quad, ab and shoulder workout. I use a 25# med ball and I really think that one move has really defined my shoulders.
I also used a med ball just as a weighted tool to use for step ups, ab exercises, squats, squats and presses, and push ups. I even run with my smaller med balls.