I don't have an iPhone but I do have an iPod Touch 8Gb. I haven't had for very long and it is a replacement for a dying (now dead) PDA, so I don't have too many apps yet.
Todo - is a task manager that I am finding very usefull.
WifiTrak - helps find open Wi-Fi hotspots.
They are the only ones I have at the moment as I don't have Wi-Fi at home, and you need to have Wi-Fi to download apps.
One thing I have noticed since upgrading from OS 2.2.1 to OS 3.0 is that the first day of the week has changed from Sunday to Monday in the Calendar.
Does anyone know how to change it back?