There are many great abdominal exercises such as stability ball crunches, captain's chair knee-ups/leg raises and hanging leg raises. There are also some abdominal exercises that you are better off not doing. Here are a few that you are probably familiar with:
1)Situps - There is a correct way to do situps but most people are not familiar with the technique. The hip flexor muscles are preferentially recruited to do much of the work during incorrect execution of situps. This often leads to muscle imbalances and low back pain. Because of the injury potential from doing situps, you are better off finding safer abdominal exercises like ball crunches. You will also get a greater range of motion when doing ball crunches.
2) Avoid doing abdominal exercises with gadgets like the "Ab Roller" and "Torso Track" because they can also create muscle imbalances. My general rule about fitness gadgets is this: YOU DON'T NEED THEM SO SAVE YOUR MONEY! You will also get better results by not doing abdominal exercises on machines. Machines stabilize your body for you and don't allow you to move your body naturally (functionally).
Of course, if you don't eat right, the ripped abs that you develop will be covered up by layers of fat. You can never leave good nutrition out of the fitness picture!