Epocrates is the program I use for looking up drug doses. I generally use the free program but am often frustrated by what it can't do. The pay version is free for medical students for the first two weeks of August. You can see the deal here:
http://lifehacker.com/5827052/daily-app-deals-epocrates-essentials-free-for-med-students-for-the-first-two-weeks-in-august
Of course, do know that Epocrates collects your prescribing habits and, I assume, sells this information.