Forearms are actually a broad range of exercises involved. Most muscle groups are of course, but forearms include everything from curls to grip strength to finger strength, wrists, and so on.
Ironmind.com has a bunch of neat ideas about developing grip, wrist & forearm strentgth, but they end up costing a lot of money if you don't watch out.
A fantastic website concerning overall grip strength & all its uses is dieselcrew.com. These guys are fucking nuts. In the archives section of articles, there's a good article on how to fashion your own forearm workout gear without having to pay the arm and a leg that companies such as Ironmind want to leech from you.
Anyway, forearms can be built up by doing curls, reverse curls, and the like. But there are other ways to hit them, too, including wrist exercises such as wrist curls & reverse wrist curls. Hammer leverages involve holding hammers or sledgehammers or offset db's, or anything that's imbalanced weight that you have to struggle to leverage, and doing wrist-curl-types motions with them.
Thick bars & thick dumbells, which instead of the regular ~1-1/16 inch handle for holding, have 2-inch thick (or even thicker) handles, force you to grip the bar harder, which really does force your grip to become stronger. If you had a thick bar, you could always be working your grip by using it everytime you do bench press, barbell/db rows, even squats, etc.
Ironmind & other companies sell grippers that you squeeze hard to close. By closing grippers for reps or holding closed for times isometric contractions, you make your grip stronger & help build overall forearms & forearm strength.
There are also Wrist Rollers which develop both grip and act as wrist curls & reverse wrist curls.
Pinching exercises incorporate the thumb directly, unlike regular gripping exercises. By pinching objects with the tips of all 5 fingers (or any combination) of each hand, you force your pinching strength to grow. Holding 2 Olympic plates together in one or both hands, for time or reps, is a good pinching exercise. Holding an Olympic plate by its center hub, and picking the plate up just by holding the hub, is another great pinching exercise.