I'm surprised no one mentioned this doohickey:
http://www.amazon.com/Dynaflex-Sport...7774019&sr=8-1
Granted, it takes a bit of dexterity to get going, so may not be best if you are having problems already, but this puppy can give your hands/fingers/wrists/forearms a good burn all around, and I for one find it more fun than some of the other products mentioned.