GripSaver Plus from Metolius
Specially developed exercise ball to strengthen and stabilize your hand and finger muscles, wrists and forearms. What is special about this GripSaver is that it trains both the contraction and the extension of the hands, so that the muscles are trained in both directions. The ball is effective for both injury prevention and rehabilitation if you should have been so unlucky to injure your fingers, wrists or elbow.
Choose from 3 different strengths:
Blue = soft
Red = between
Yellow = hard
Step-by-step instructions included.
Also see Metolius' own guide here.