Sitting on a balance ball requires good posture, encourages movement, and involves your whole body. It will help strengthen your core stabilizer muscles, your upper and lower back, and your legs.
I have replaced the great chair I have at my desk with a balance ball. My posture seems to be improving even though I stay in front of the computer countless hours. Fully recommend it.
Beware: it will hurt like hell for the first week or two.