I really don't believe we evolved to sit all day long. Sitting creates tight hips and hamstrings, causes a tight neck and shoulders, and often leads to back pain. If you suffer from these ailments, ditch your regular office chair and replace it with an exercise ball ($27).
When you sit on a firmly inflated ball, you'll improve your balance and tone your core muscles from your desk. It allows you to focus on keeping your spine long, and you can even use the fitness ball for wall squats or other exercises during the day. Sitting on a ball has also been proven to help kids focus and do better in school. If it works for them, it'll work for you, too.
If you're worried about the ball being too roly-poly, and not having a back support to lean on, then check out Gaiam's BalanceBall Chair ($99). I know it's kind of pricey, but if it can prevent discomfort while you work, then isn't it worth it? To see a video of this ball in action