There used to be a link on the EN World website to a company that made 3E D&D programs for Palm Pilots. I don't know if those can be used for other d20 games, or if they make programs for other games.
It would be a big help, though, for figuring things like speculative cargo prices and space travel calculations.
Back in the mid-80s, when we first started playing Traveller, one of our players had a programmable Texas Instruments calculator. We wrote programs on it for a lot of the calculations we used frequently in the game. It worked pretty well, and saved lots of time. Our merchant character was always using it as his hand comp, using it to calculate how much money he could make on various cargoes.