If you are flying international the following apps are great. Also, I carry a D-Link Pocket Router (around $37 on Amazon) and set up my own secure wifi hotspot in my hotel room at each layover. I turn on my Droid in wifi mode only and it connects to my network so I can use Skype on my phone. Works great and I don't have to carry a headset for my computer any more.

- Skype - download and use the Skype app from the Android MarketPlace, not the Skype Mobile that comes on the phone.
- ZZTrop - calculates rest periods for 3/ 4 man crews.
- Currency - great app for current currency conversion. Use the wifi in your room to update the rates right before you go out. You can input any currency you want and convert both ways.
- All Subway Pro - shows subway maps from all major cities in the world. Even some you wouldn't think would be available. Once you load the app, pull up the subway maps you will need and from then on it doesn't need any type of connection to view those maps. It stores them on the SD card in your phone. Great app.
- Google Translate - Great translation app but does need a connection to work. You can translate and store commonly used phrases to use offline.

Other apps I really like:
- App 2 SD - moves apps you download to the SD card if you want.
- Barcode Scanner - makes downloading apps painless.
- Color Flashlight - makes finding a lounge chair in the hub easy!
- LED Flashlight - if you want a really, really bright flashlight!
- Dolphin Browser - much better web browser than the one that comes on the Droid.
- Handcent - much better texting app than the one that comes on the Droid.
- Night Ringer Pro - great app to use while you are sleeping. Silences all incoming calls / alerts except for people in your contact list you designate as emergency contacts. It will let those calls / texts ring thru.
- Pandora - gotta have Pandora!
- Tip Me - app that calculates any tip amount (including or excluding the tax) and will split up the bill any way you want between however many people are eating with you.
- Poynt - very good app to find almost anything near you, Restaurants, businesses, lowest gas prices, movies, people. Once you find what you are looking for you can map it from your current location from within the program. I use it a lot on trips.

I've got more apps on my phone but these are the ones I use the most. A great website to checkout apps is: AndroidZoom - Download thousands of Android apps from the Android Market. You can search apps and see what others have to say about them. Good luck with the Droid. couldn't do without mine.