Meet the OmniBall: a total body trainer that you can use with your hands and feet to work your core and add instability to your favorite workouts. These are available in 2lb and 4lb versions. They come with velcro straps, so you can use them as gripless weight.
The OmniBall is ready for strength, flexibility, and stability training. It features a 360-degree omnidirectional rolling system. You can find out more about it here.