Not very many years ago, the elliptical machine was the new machine on the block. Now you see them in every gym, and you're lucky if you can nab one. That's because the elliptical burns as many calories as running (700 an hour), but without the stress on your knees and feet. For a low impact workout with a high impact on calorie burning, the elliptical totally trumps the treadmill.

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Biking

It's no wonder bicycling is such a popular activity for fitness: it's actually fun. There are few other workouts that deliver the same punch -- 650 calories an hour burned -- and feel so much like recreation. Whether you cycle in the great outdoors or hit the virtual trails on your gym's stationary bike, you'll have a session that delivers fun and fitness in one.

Power Walking

If running is too hard on your heels and joints, you still don't have to settle for a slow-burning walk. A power walk can burn up to 600 calories an hour, which still puts regular walking to shame when it comes to numbers (a slow stroll only earns 280k). The trick is to get your arms moving, and keep a quick pace. The extra speed means extra burn, and the arm movement helps you have a full body workout to boot.

Rowing

It's hard to believe that a machine you can sit down in can be such an effective fat burning mechanism, but that's exactly what rowing can do. The secret is that it targets some of your hugest muscle groups, most notably your back. Your arms, shoulders and legs get a workout too, and your abs reap even more benefits, all to the tune of 550 calories per hour.