Here's a practical design: the Hydro Physio Lifestyle Water Resistance Treadmill (believe it or not, that's the short version of the name). It combines the water resistance of swimming with the monotony of running to create a really strange form of exercise. The water level can be adjusted so it focuses on different areas during your workout, and, to be honest, it does look like a unique way to get your exercise in, despite how crazily impractical it is. [Product Page via Oh Gizmo!]