While the Speedlimit plugin allows you to restrict the velocity of an object in your scene, the Limiter tool gives you the ability to restrict the position of objects. You can prevent objects from exceeding different positional constraints such as dropping below ground level, even if your keyframes pass beyond this level.

Since the limits are fully animatable, you can even add or remove constraints during the course of your animation.

This is just one of over 20 plugins in the James K. Polk Plugin Collection.