A huge part of the delight in traveling abroad is in the small things, like finding out-of-the way sites that only the locals know. A good trip planner keeps track of everything on your must-see list. If you have to pick a single app, TripIt provides integrated trip management, and it's free (an add-free upgrade is available for $4). It reads all the confirmation e-mails you send it, pulls them apart and puts all the pieces together in a really useful way. It also lets you share itinerary details with friends and family. You save time and energy you would have spent trying to keep up with your itinerary and confirmation data. It's like having a personal assistant you never have to pay.
No mobile app can be completely good at everything, and there are a couple of other general travel apps you might consider based on your priorities. For example, Trip Advisor's mobile app is famous for its highly regarded reviews of restaurants, hotels and airlines. Another contender worth your attention is price-aggregator Kayak, which searches across a multitude of sources to track down rock-bottom rates for flights, cars and hotels.
Let these economy-focused apps work for you, and you'll always have reasonably-priced adventure at your fingertips.