Half Bosu Ball
The half bosu ball is a great piece of equipment to add to your workout routine. I love it for 2 reasons.
The instability this ball creates makes your body use more muscles than it normally would in each move, particularly your core. I can assure you it will always keep your core engaged.
2. Extra cushion.
When doing any type of plank-related move the half bosu gives you some extra cushion, taking some of the pressure off your wrists and shoulders, while still making your core work harder due to the instability factor. I'd rather do mountain climbers on a half bosu any day than on the floor for this reason.
The one negative is that it's not very portable, however, I have found that when traveling most hotel gyms have one.