A moving belt, powered by an electric motor, on which you can walk or run. Raise the bed or increase the speed to increase workout intensity.

Pros: No learning required. Provides training for runners and walkers. Most likely exercise machine to be used, not abandoned. Some models fold to save floor space.

Cons: Lower-priced models may not be well-constructed and are generally not suitable for running. Nonfolding models take up as much space as a couch.

Price: $300 to $700 (walkers only); $800 to $1,800 (walkers and occasional runners); $2,000 to $3,500 (serious runners).

A combination of a stair climber and a cross-country ski machine that you stand on and pedal. A variable-resistance flywheel adjusts the difficulty of the workout.

Pros: No impact, so you're less likely to injure joints than on a treadmill. Lower perceived exertion for equivalent workout intensity. No motor, so less noisy. Handles provide modest upper-body workout.

Cons: Lower-priced models may not be well-constructed. None fold; many take up as much space as a couch. Tall users can bump their head in a low-ceiling room.

Price: $500 to $1,000 (feel flimsier); $1,100 to $2,500 (approach the quality of those you'd find at a gym).