Weighted balls aside, I recently sat in on a pitching / hitting seminar where the coach recommended med ball throws as a method to generate immediate velocity gains.
Looked like this, minus the funny ramp thing...
Not that you are strengthening the arm but forcing the body to engage the legs and core more to pitch (or to swing the bat, same torquing motion really).
I would love to put my son on the gun, have him throw a few pitches. Then do a set of med ball throws, and radar a few more pitches after that.