I like my swiss ball. Picked it up for 12 bucks, and it has done a damn good job in the short month Ive been using it. There are a ton of things you can do with it, including adding weight. My favorites are to get into the pushup position, toes on ball, and bring my feet+ball closer to my hands. Basic crunches. and putting the ball between my legs and lifting the ball into the air and coming up and touching it with my hands.
It gives me better cordination and balance too, so thats pretty nice.