My key travel apps for iPhone (prices as of July 3, 2009):

1. TravelTracker with TripIt $7.99If you take more than 6 trips a year get this app and sign up for TripIt service on the web. TravelTracker shines over the free TripIt app because it keeps past trips and has many more features. Silverware is a good company and this is a good app. Loved the previous TravelTracker incarnations on the Newton & the Palm, and I got it on iPhone as soon as it came out as a new-from-ground-up, smarter app. I’d hold off on getting the “Pro” version for now though… there are some iPhone 3.0 issues to sort out. In a few weeks I will be upgrading to TravelTracker Pro though, so I can get live flight status plus download and upload to TripIt. Update: A new version of TravelTracker posted to the App Store today—July 3, 2009.
2. TripIt for iPhone FreeHuh? You just said TravelTracker. I use both. The TripIt app is less cluttered and faster— for the current trip, but I can not easily forward an itinerary or do many other things that TravelTracker does so well. I use TripIt for quick access to info on the current trip, like hitting the rental car info with my big fat thumb as I dash away from baggage claim.
3. The Weather Channel FreeAs National Geographic puts it:
   

   [The Weather Channel app] gets the nod for customization and the ability to check out conditions in multiple locations at a glance.

4. AroundMe FreeI admit it, Starbucks pwns me. AroundMe helps me quickly find that essential latte, or a local equivalent and other conveniences (ATM/bank, restaurant, pharmacy, retailer). I’ve tried coffee-specific apps and a few brand-name bank/retailer/restaurant finders. This particular app works well and seems current most places I go, and does away with having a slew of category/vendor-specifc apps.
5. Currency FreeFantastic currency converter that auto-updates exchange rates. Shows a purchase amount in multiple currencies of your choice at the same time. Easily switch from entering in one currency to entering in another. Very helpful on my trip to Singapore when trying to figure out what $5 USD was in SGD or what $900 SGD is in USD (answer: 1 nice watch).
6. Taxi Magic FreeGreat app for getting a taxi when you aren’t in an urban center or near a taxi stand. It electronically hails a taxi and sends your current location to your choice of cab companies. You can even create an account & use it to pay for the ride, without using cash or credit in the cab. Cities supported include: Atlanta, Austin, Boston, Charlotte, Chicago, Dallas, Denver, Kansas City, LA, Minneapolis, Nashville, NYC, the OC, Pittsburgh, Phoenix, Portland, Sacramento, San Antonio, San Diego, Dan Francisco, San Jose, Seattle, St. Louis, and Washington DC.
7. Urban Spoon FreeThe best way to break the logjam of “I dunno, where do you want to go.” Also nice to find something interesting & local instead of the shopping mall-perimeter chain restaurant defaults.

Then there are the airline superstars of iPhone web app-friendliness. The standout here is JetBlue, who has a very useful and colorful web app. Honorable mention to Southwest Airlines, who made great use of the very cool iUI iPhone templates found on Google Code.

To make the most of any of the web apps below, just visit the URL on your iPhone and then bookmark with the “Add to Home Screen” option.

Great iPhone web apps from airlines to add to your iPhone homescreen (actual iPhone/Touch home screen icons shown):

• JetBlue mobile http://mobile.jetblue.com/
• American Airlines mobile http://mobile.aa.com/
• Delta mobile http://mobile.delta.com/
• Southwest mobile http://mobile.southwest.com/

I heard back from @Alaska Airlines on twitter that Alaska is working on it. Go Alaska! No word from United Airlines on when http://ua.flightlookup.com will become more iPhone-aware. Sadly, the very-cool, but Flash-dependent site of Virgin America doesn’t make the iPhone cut either (as of July 3, 2009). Btw, lots of airlines are on twitter, as I found out working on @AICC followers.

A Few Extra Travel-related Apps
Rounding-out my travel page I have Aeroguide Lite (Free) to recognize things like that rare Ilyushin Il-62 I once saw over SFO (an Il-62 looks like MD-80/DC-9 with 4 engines or a Vickers VC10).

Other miscellanneous items on the page include: Gas Cubby ($9.99) for recording and charting the MPG, etc. of my own car; FedEx Mobile (Free) for help with shipping; and Google Earth (Free) for miscellaneous mapping and geographical visualization.

The National Geographic Adventure List
To get more detailed information on their picks, please see Travel Tech: Top 20 iPhone Travel Apps from National Geographic Adventure. I’ve add my opinions to their list below where I had feelings one way or the other. For reference, here is their list:

1. Next Flight $2.99
2. Urban Spoon Free
3. HearPlanet reg $5.99 (on sale for $1.99 on July 3, 2009)
4. Air Sharing $4.99(Nix I say, get FTP On the Go)
5. Tweetie $2.99 (Nice, but nix say I. Get free TweetDeck or Twittelator Pro for $4.99, or both as I did)
6. IAmHere $0.99
7. World Customs $0.99
8. Wi-Fi Finder Free
9. The Weather Channel Free
10. Google Earth Free
11. Packing $1.99 (on sale for $0.99 on July 3, 2009. This one *I* might try)
12. Room Free
13. FlightTrack Pro $9.99 (Nix, I say, get TravelTracker w/TripIt)
14. Lonely Planet Phrasebook $9.99
15. Skype Free (I totally agree here–a very popular app, 4M downloads & estimated to be on 10%+ of iPhones)
16. WriteRoom $4.99
17. (Amazon) Kindle for iPhone Free (I totally agree here too)
18. Cheap Gas Free
19. Babelingo $3.99 (on sale for $1.99 on July 3, 2009)
20. Where Free