Stair Climbers
Here's a tip - if your gym has both the classic, two-step Stairmaster-type machine, and a Stairmill, which looks like a revolving set of stairs, go for the Stairmill. The Stairmill burns more calories because you're actually lifting your body weight up as if you were on a real staircase, as opposed to just pumping your legs up and down on the Stairmaster (if you have weak knees, though, the Stairmaster is the better choice). No matter which type of machine you choose, stair climbers are a great cardio workout that can add a bit of tone to your legs and rear end (this does not absolve you from doing lower body workouts, however!). But stair climbers have one big problem - a huge cheat factor. The moment you start leaning on the handrails, you are not getting all the workout you should because you are no longer moving with your full body weight (why do you think they call it a "weight bearing exercise"?). As a result, you are not burning as many calories. The more you lean, the fewer calories you are burning, and the less effective your workout is. Touch the handrails lightly, just to maintain balance and let your lower body do the work. Maybe you won't be able to go as long as when you hang onto those handrails, but that means you are getting an equal workout in less time (never thought of that benefit, did you?).

Elliptical
Here's a very low-impact, weight-bearing machine that, if it has moveable handles, engages your whole body. No wonder the elliptical became many exercisers' favorite machine the moment it started hitting the gyms. Because they are so low-impact, however, they are not the best bone-density builders in the gym and, believe it or not, it is easy to overtrain on an elliptical! Because of the smoothness of motion it is easy to forget just how much you are exercising and you can overdo without even realizing it. Of course, this ease of motion is also why so many people have gravitated towards it. While it's fun to fly on these machines, do crank up the resistance now and again for a more intense workout.

Rowing Machines
These tend to be the least popular cardio machines, and these days not every gym has them. This is too bad, because they are great full-body exercisers. Contrary to popular belief, they do not rely solely on arm work - rowing machines engage both your legs and your arms. They are low-impact machines and really quite fun, if you give them a chance. Good rowing machines have the same smooth feel as an elliptical, only from a different perspective.