Fad diets. Right now HGH and HCG are the rage but any fad is a waste.
Items sold to you. Diet programs that try to sell you products (cookies, shakes, etc.) are a waste of money because you learn to eat a cookie and not real food. Even if you reach your desired weight, you have not learned how to eat properly, and will probably gain all of your weight back (upwards of 95 % of dieters do). Go with a diet that teaches you to eat real food.
Depends. Most programs work to the extent that they make you expend more calories than you ingest. Most failure because people get bored with them, do not like them or do not follow them properly. It is a waste to pay for a program which you fail to follow. There are no miracles here and nothing is easy. You need help from nutritionist, exercise and your family doctor.