In addition to ePocrates, check out Skyscape which has another (maybe better) drug database, plus a searchable disease program, medical calculations, etc.
I'm a flight sim fan from back in the Microsoft FS3 days, and I'm LOVING X-Plane: a pretty full featured flight sim right in the palm of your hand.
Wings is fun too for quick casual flying around in imaginary scenery and that one is free or very cheap.