Re: iPhone "must have" apps?
A few of my fav's:
LogMeIn - Remote access your PC desktop.
Red Laser (lazer?) - Scan barcodes on items to price check websites
British Gas - Update gas/elec readings to your account
EveryTrail - Great to use when going for a run/ride. Uploads to a website, where you can see a graph showing your speed, average speed, distance etc.
AroundMe - Finds hotels / petrol stations etc
Angry Birds - addictive, very.
Worms
Moto X Mayhem
Sim City
Talking Carl - funny app, repeats everything you say in a funny voice
LogMeIn screenshot: