Star Walk - point it at the night sky, it maps the stars out using GPS and orientation and tells you what they are, tap on them for detailed info.
Living Earth - Picture of the globe, you can swivel it and zoom in a bit (but not on cities and stuff like Google Earth) - the cloud formations are real, updated from satellites every 15 mins. Been watching the earthquake in New York recently.
Usefulness? Zilch, but very very impressive and pretty.
Cut the Rope is the best game on it - even better than Angry Birds.
Sky Go - watch sky tv live on your phone
Sky+ - set your sky+ box from your phone
Met Office - weather
Train Search - by XCountry trains but gives live updates to all national train services plus timetables etc etc (v. useful)
Endomundo - sat nav exercise tracker
Paddy Power - betting
Racing Post - betting/form/results
Tapatalk - forums
Angry Birds
Lightsaber
FlickGolf!
iGunLite
Bump - bluetooth files to other iphone users just by shaking your phone at them