This is the exercise ball.  This is a cheap and effective product that when used in conjunction with your normal body weight exercise routine (such as pushups, situps, lunges, etc.) will take their effectiveness to another level.  It forces more focus on the desired muscles for the particular exercise and adds another level of diffculty to the exercise: balance.  Because you must balance on the ball and prevent it from rolling it results in added stability and strenthening as well as turns any standard exercise into a core strenthening exercise.