Killer apps for me:
Cams Ahoy (paid for but doesn't use data connection) and Trapster (free but needs a data connection) - speed camera notification systems that work in Switzerland.
WorldView -webcam viewers - good for checking snow conditions
Starmap - maps out night sky
Deezer - a bit like Pandora
TV Plus - UK tv guide (for Sky users in CH)
Midomi - better than Shazam in that you just have to hum or sing the song for it to be identified.
Brain Challenge - the only game i play on a regular and continued basis