Have you had a mooch at the Apps. store yet? There's a handy little free one called AroundMe that finds your location and lists local petrol stations, restaurants, hotels, cafes, pubs etc..
Probably quite handy if you're pissed and lost as it will direct you to the nearest pub. I love my iPhone.
Yeah put that on along with all the obvious ones so far like the facebook, myspace, ebay ones etc and the trains one which is especially useful as it tells you the train times, whether their on time, the expected arrival time and even the platform number.