I don't use a computer when running the game. I do use a computer when before and after the game, though:II'm generally thinking of programs, tools or resources that are actually used during the session, but something used outside the sessions is probably equally useful and relevant to this thread.
Virtual Combat Cards (Virtual Combat Cards D&D 4th Edition Initiative and Combat Tracker | Ex Nebula) - I find this really helps combat move along fluidly. Although I do wish you could store a list of effects and conditions in it.
* Microsoft Word, which has the adventure I'm running in some cases
* Microsoft Excel, which I use to track combat HP and initiative

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.