Well-maintained, commercial/gym quality Matrix S1X Stair machine. I've been the only user since purchasing it refurbished from Johnson Fitness.

As most of you know, it is a terrific, low impact, cardio workout that will augment your cardio fitness and training. Historically, alternating my training between long-distance runs and stair workouts helped me through four marathons and a handful of half-marathons and 10K's.