With my iPhone being just a month old, here's some of the apps that I have found the most fun/useful:
Contagious fun that has me beating my finger weekly trying to win the weekly challenge, included in Blitz mode, that has a nice score sync to fb.
Nice to have the ability to blog on the fly, some features have yet to be added - notably adding an URL into text but overall a handy app for blogging away from home.
Diary app that ties in fb, Twitter, Flickr and Last.fm social feeds with your your own 'moments'. I can see more feed options being brought in during the course of future updates. Very nice way of bringing all your activities together into a modern version of a diary.
These early impressions make my decision to get an iPhone rather than the newer model BlackBerry, a good call. I just hope multi-tasking is an add placed into iPhone's OS upon the release of 4.0.