It's not REALLY an app. Rather, it's a web site that is custom designed for your mobile phone.
Go here on your phone's browser:
https://www.defensetravel.dod.mil/mobile/
This let's you do everything from look up city-pair government airline fares to per diem rates. Very useful if you are planning a trip and need to give an estimate to your boss!
More information here: https://www.defensetravel.dod.mil/site/news.cfm?ID=16
