New workout.

This workout is already popular, but I had never heard of it. I forget the name now, but who cares. You get your upper body in the push-up position, and pull your knee into your chest (gently) and either rotate, or do 5-10 at a time with each leg. You get the same effects as running up a steep mountain. Apparently it helps your core, legs, and it definitely gets your cardio going a little bit too after a while.

The only downside: your feet start to slip after a while.
Very annoying.
:)