If you could install only one app to the iDevice (apart from the preinstalled) which one would that be?
And to make things more fun, post also why the poster above you has choosen a stupid app.
To get things started here's my pick:
Battle.net Mobile Authenticator.
It generates a code which let's me log into World of Warcraft. This code is a way to stop account theft. Without the app I couldn't log into my WoW account. My life would cease to exist and I'd die on the spot. Well, actually I'm not playing as much as I used to and havent logged into my account for 6 weeks, but it's funnier this way.