Arm ergometers are like upper-body bicycles -- you sit or stand in front of the machine, hold on to the handles, and crank your arms in a circle, just like pedalling a bike with your hands. They're an excellent option for people with lower-body injuries, or for anyone who wants to improve upper-body muscular endurance.
However, you'll burn fewer calories using an arm ergometer than practically any other piece of equipment on this list because you're focusing solely on the smaller muscle groups of your upper body. The more muscles you engage during exercise, and the larger those muscle groups are, the more calories you'll burn during a workout. If one of the primary goals of your cardio routine is calorie burn, then the arm ergometer just doesn't measure up.