In reply to Mountain Lass:
I think you have several options, all based on some kind of ball.
Metolius Grip Saver - I have no experience of this but others may. Look it up.
Gyroscope type balls - you keep the gyro spinning by cyclic movement of your hand/ wrist. I have one, it gets you boxed too quickly and is really only working the wrist not the fingers. - also definite snigger potential (gender dependant)
Putty - available in various viscosities for you to squeeze and pull in a variety of ways. I used it to get back into form after a hand injury - definitely worth a go.
Post edited at 13:38