At the top of their list was Next Flight. For a mere $2.99, you can track flights from over 4,200 airports and over 1,000 different airlines. Coming in at number two is UrbanSpoon, which they called "the best guide to local restaurants yet," and best of all, it's free. HearPlanet cost a little more at $5.99, but it alerts you to nearby attractions and plays their Wikipedia pages out loud. Need to access your personal HTML webpages, PDFs, text, and other files at any given time? For $4.99, Air Sharing allows you to do just that. Number five is Tweetie, not necessarily a travel app, but it is top-rated for using Twitter on-the-go and costs only $2.99.
For just $0.99, IAmHere lets your friends know exactly where you are on Google Maps. World Customs (also only $0.99) provides you with international dos and don'ts. Wi-Fi Finder does exactly what the name says in over 135 countries and it does it for free. Rounding out the top ten on Nat Geo's list are The Weather Channel, which is free and also pretty self-explanatory, and Google Earth, another free way to figure out where you are and where you want to get to.
As for the rest of the top 20, they are: 11. Packing, 12. Room, 13. FlightTrack Pro, 14. Lonely Planet Phrasebook, 15. Skype, 16. Write Room, 17. Amazon Kindle, 18. Cheap Gas, 19. Babelingo, and 20. Where. For more detailed info on each app, visit NationalGeographic.com.