KAYAK App is actually better than the website! Love this all-in-one travel app where you can compare prices on flights, hotel and cars, then book them straight from your phone. Also keeps your itinerary handy for you!
Google Earth Even before you decide where you're going to go, download Google Earth and do some pre-exploring. I rally like the 3d map view, although I miss the 1st person street view.
TripAdvisor Hotels Flights Excellent app for checking reviews on hotels and finding the best prices on flights. Real user reviews are key when booking travel in places you aren't familiar with, and I also like the Facebook connection.
World Travel Guide by Triposo Really and incredible and useful app for traveling internationally. Check local restaurants and sites, and find some off the beaten path locales. What's really great are the offline maps that don't need wifi to use. There are also city specific versions as well.
Europe Travel Guide A really basic, but really helpful travel guide for Europe. Great suggestions on places to go and local restaurants.