As we're spending more and more hours in front of the computer, how we sit can make a big difference. Pain in the neck, hands, and back are very common results of poor (and too long!) posture. The results of poor posture are immediate suffering and sometimes long term physical damage. Not to mention reduction in productivity.
Several factors that affect our "computer posture" are the choice of furniture (computer desk and chair), and computer accessories (keyboard, mouse, etc.), the computer placement (e.g., how far the screen is from our eyes), and sitting habits.
Ergonomic design guidelines can assist in making the right choices (Wikipedia: "Ergonomics is employed to fulfill the two goals of health and productivity. It is relevant in the design of such things as safe furniture and easy-to-use interfaces to machines."). See, for instance, Ergonomic Guidelines for arranging a Computer Workstation - 10 steps for users.