whale boat rowing!!!.....I started training for this a couple of weeks ago and my forearms are fried after every practice. It obviously works more than just forearms, but the most noticeable burn is in the wrists and forearms.
i'm having good results just from picking up a set of dumbells and doing 2 laps around my gym while holding them at my side a couple times a week,most exercises work your forearm anyways, and many peoples forearms are probably overtrained