Once you've used TripIt you'll never turn back. Download the app prior to departure, and it will scan all of your confirmation emails and compose them into one nifty itinerary. Bonus points for the fact that it will add also import dates and times into your calendar and sync maps along with your itinerary so you're never left fumbling around looking for your hostel.
There are thousands of free destination guides out there, but I find none are so comprehensive or so easy to use as Tripomatic. This is a one stop-app for almost every major city you could think of, and is filled with the typical tourist traps plus some more insightful local additions.
Sometimes you just want a rest and have someone else plan your day for you: if so look no further than Viator. Simply pop in the name of your destination and be greeted with hundreds of day or multi-night trips and tours. You can book straight through the app and almost all allow you to present e-confirmations, so no fussing around with printing out tickets while you're on the road. This is also a great app for filling in free days at the last minute.
4. XE Currency Converter
It may seem like a rather boring addition to the list, but a trusty currency converter is a must-have for even the most basic of overseas trips. After much trial and error XE Currency Converter has become my go-to.
5. Your airline app
Most airline's today have their own free app, which will allow you to import your flight details, check departure statuses, check-in for your flight and store your e-tickets. Some even have added features (Virgin Australia's domestic app for instance allows you to hook up to free in-flight wifi!) which will make you the savviest flyer on board.