As Andy mentioned, Wacom. They are the best hands down. The graphire series is for the home user, the Intuos is for the professional, an the Cintiq is just awesome. I used an Intuous for years because I had very bad tendonitis in my wrists from competitive powerlifting and working 8-10 hrs a day on a computer did not help my situation any. The natural ergonomics of holding an pen or the airbrush tool was much more comfortable. I recently upgraded to a Cintiq 21UX display and I couldnt even imagine going back.
Another awesome tool to increase your workflow is the Logitech NuLooq dial. It retails for a supposed $80 USD although I found one online for $55 by using Froogle to search for it.