Why you want one: A stability ball is a versatile tool that you can use to build strength, increase flexibility and balance, and challenge your muscles in new ways.
Try this: The TRAINERball ($29.99 at Trainerbrands) has 12 exercises printed right on the ball's surface, making it easy to learn how to use. (You can also find stability balls for about $20 at Target and Wal-Mart.) Be sure to buy a ball that is the right size for your height: 55 cm for people 5'0"-5'5"; 65 cm for those 5'5" to 5'11" and 75 cm for those 6'0"-6'3". Most balls are burst resistant up to 400 pounds.