I recently took a two week vacation to England and Spain, traveling with only my iPhone to help keep me organized and to guide me along the way. I firmly believe that the key to a great vacation is to leave your laptop at home.
In preparation for this trip I found a great iPhone application for organizing all of my reservations, train ticket codes and other documents. File Lite helped me take all of my information with me in the form of word docs., pdfs and text files. The reason I opted for this over using notes was to have a PDF version of all my receipts and reservation confirmations. File Lite works wirelessly over any network and you can connect to it via an IP address that pops up wherever you are connected. My think was that in a worst case scenario I could pull these files off my phone and print them again. (Of course I had a hard copy that I used as well). Overall this app was incredibly easy to use and it was the 2nd or 3rd I tested before leaving.
The second app I want to mention might seem obvious but I feel the need to restate its benefits. The Skype app for the iPhone is great and making calls with it is unbelievably cheaper than using your international phone rates. I spoke to someone back in the States from Spain for two hours and it cost about $3. (This would have been a $120 call on my normal plan). The weakness of this app is that you have to have a Wireless signal and typically a pretty good one. I did get by at times with a weak signal but you run the risk of dropping the call. The integration is good too, Skype allows you to pull in contacts from your normal address book. One more thing…I used my desktop version Skype to book 100% of my trip before I left.
Last but not least I highly reccomend the Subway and Transit maps developed by Presselite. I installed both the London Underground and Barcelona Subway applications before leaving and they were essential to my not getting lost. These apps do not require any Wireless or Cellular network (but do use them when they are available) and have really helpful trip planner/route finder features. Type Presselite into your iTunes store and see the variety of maps they have.