The elliptical trainer is the most popular piece of cardio equipment in the gym. (fitwirr.com)
With that said, you could use any other piece of cardio equipment providing you can do those intervals of high intensity followed by recovery at a lower intensity. (gymjunkies.com)
There are probably some great pieces of cardio equipment you can use (without just going with the default treadmill or elliptical machine). (rawbarrel.com)