A new twist on an old classic. The straight bar wrist roller has been around for decades, and is an excellent all around grip and forearm builder. By adding steel spheres to the ends, the motion becomes more of a supination/pronation movement, rather than just the working the flexors and extensors. In simpler language, it torches your forearms from an entirely different angle.
One interesting (and effective!) variation is to reel in a sled with a wrist roller. This allows for a much longer cord to be used, prolonging the exercise (and the intensity) to produce even more amazing results.