I know, it looks silly, but don't laugh at me just yet! I've taken enough grief from co-workers over the years, anyway. But I prefer to stand up a few hours of my work day while on the computer. It may sound odd to some, but it's actually very comfortable, and not an unheard-of practice in the working world. I like to switch back to sitting after a while, though, as it's not good for the back to stand absolutely still for prolonged periods either. My solution is a workstation that can go both ways. As you can see in the picture, I use a file box, stepping stool and photo boxes to achieve the right height for monitor, keyboard and mouse. It takes me about 15 seconds to switch from one to the other. The benefits of standing while working are better alertness, reduced strain on the back and neck, and even enhanced functioning of the lymphatic system (really, look it up!) It also has completely eliminated that 3 o'clock dip in energy for me. If you use a notebook computer, this system is also easy to achieve with a height-adjustable laptop stand (this one goes up to 39").
Happy typing, worker bees!