I do wrist rollers, and pinch block holds... But, the forearm will normally get a good workout from bicep related stuff... Like LV said, throw in some hammer curls and wrist curls... Or, even some reverse curls, supination/pronation.
Forearm specific is not necessary unless you are pinpointing martial arts training or rock climbing, etc...
Try slamming your open hands into a bucket full of sand. Then close your fist completely and re-open it in the sand.
Checkout this site:
I have the grippers (trainer and #1), pinch block, Titan's Telegraph, Eagle eye loops, and wrist roller... I use these tools at the beginning of climbing season to cautiously get my tendons back up to speed.
-Rip