Depending on how much travel and who you fly with then, KLM; BA and the OneWorld alliance all have useful apps for airline travel.
WorldMate is good if you do travel lots and need hotels etc as well a flights.
AeroWeather is a great app. Hurricane, PilotWiz and Measures are all pretty useful, as is the Met Office App. Snowtam is also pretty useful.
World Time is a handy app if you have to call/travel round the world lots.
iHud and Flight Control are good for a bit of fun.
