If you need a place to hold your travel materials [tickets, itineraries etc.], Trip Case is one such app as is Tripit’s [which has an online component]. You can also transfer to your iPhone or iPad documents in PDF for later use, either in preparation for the trip or while on it through FileApp, Air Sharing, Dropbox or Evernote or such cloud resources.
Other helpful or even essential apps are those for Translations, Currency Conversion, Unit conversions of all kinds, scanning barcodes and QR codes, apps which give you a flashlight or other utilities. For the latter especially I like AppBoxPro, which comes in a free edition [AppBox] and also a paid one, which I have and use. There are plenty of fine free apps for these uses, though.
For conversions I've been using Convex which has all sorts of unit choices [radioactivity, speed, temperature, time and a whole bunch more] and which is great for the visually/graphically inclined. Of course, my favorite [AppBoxPro] has all those functions as well as many others.
[Again, search for the above names of apps to find them in the iTunes App store]