i always liked the exercise where you put a rope through a stick, tie a weight to the rope and roll it up and down (Not using your arms of course), but for the life of me, i can't remember what they're called!! (someone help me out here)
For grip strength, try gripping a couple of 10lb plates together in each hand so the smooth sides of the plates are on the outsides. Hold as long as you can until they drop to the floor.
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All of these suggestions are good, but I'll tell you what shreds your forearms like nothing else - hitting. I coached baseball the past four seasons and didn't get to the gym as much as I'd like. I'd lose strength and size in every other muscle group but forearms. After a few weeks of hitting infield/outfield to the kids they'd be jacked.