Since most people’s iPhone’s aren’t unlocked and AT&T charges an obscene amount for international data roaming, I’m going to assume that the majority of users aren’t going to be able to use any applications that need to access the Internet. Therefore, I’ve divided my list of iPhone apps in to two categories: Offline and Online.

Offline (no Internet required):

• GoodReader – An offline html/doc/pdf reader and my #1 app. I have 3-4 different websites backed up on my iPhone, along with countless Lonely Planet pdf files. It’s because of this app that I don’t travel with a guidebook.
• Currencies – My personal favorite currency convertor (except that it doesn’t include Syrian pounds and Lao Kip)
• Babelingo – A travel phrase book with 300+ phrases in 11 languages (but still no Arabic…).
• MotionX GPS – An offline GPS application. When I know I have an upcoming bus or train ride, I often store the destinations as gps waypoints so I can monitor how much longer I have on the journey.
• gUnit – The most comprehensive Unit Convertor for the iPhone
• Your favorite time-wasting game: My current favorites are Catan, Spades Classic, Adikus’ Backgammon and Lux DLX

Online:

• Kayak – The easiest way to check flight prices on your iPhone
• Flight Status – Great interface for monitoring/tracking flights
• Skype – Great for a quick and cheap call home when you find free wifi
• Tweetie 2 – My personal favorite iPhone Twitter client (the latest version supports geotagging tweets)