Finder. I want to be able to get docs on the phone and store them. Not many with the limited space, but a few. It would always be nice to have my resume on there and then you could e-mail it whenever.
Definitely would love to be able to store files on the iPhone, I used to use my old iPod mini as a portable drive and it seems like such a waste if I can't store docs on my iPhone.
Also, I'm a chem student and I pretty much go to wikipedia about 20 times a day looking up atomic masses, densities, heat capacity, atomic radius etc.
Now maybe it's just because I'm a teeny bit of a chem nerd, but a touchscreen periodic table would be the most awesome app ever. You click on an element and all its info comes up... I can just picture it now.