As a developer who has an iPhone, I am looking for applications that are developer-centric. What iPhone applications do you find useful as a developer?
It really depends on what kind of development you are doing. Many of the samples on Apple's site are really useful for figuring out the basics.
As far as ongoing usefulness - if you are doing accelerometer programming, the AccelerometerGraph program is particularly nice for figuring out how the accelerometer responds to various movements.
Not sure this counts because it isn't a "developer" app but...
There's a free RSS Reader I downloaded the other day and subscribed to a few development blogs. It caches the most recent posts so you can read from anywhere if you don't have internet (I have iPod Touch, not iPhone).
TouchTerm is a very nice SSH terminal client, available in free or paid versions.
Programmer is an RPN programmable calculator designed for programmers. There are many others - look to see which suits you best. 99c.
YouNote lets you sync notes kept on the phone and the Mac. Free.
BetaHelper reads the device ID without needing a Mac - useful for ad hoc provisioning. Free.
ColorRef lets you store and review colour themes.
Developer's Tool Kit is expensive at $4.99, but combines calculator, colours and ANSI UTF-8 character set lookup. I've not tried it.
Jailbreak your phone, and install ssh. Your iPhone is a full-on playground in your pocket, but most of the most fun is not available to you until you can log into the device.
If you jailbreak your phone (you don't need to unlock!) you can do whatever you want.
It's a programmers toy at that point.