Navigon just released their GPS for the iPhone. Temporarily priced at $69.99 till August 15th then the price will jump to $99.99.
It's a great price if you don't want to carry an additional piece of hardware around when you have the iPhone already available to you. The only flaw is you cannot use your phone while Navigon is running.
Navigon on the iPhone works even if you do not have the data plan. It takes over 1GB of storage space. It downloads the maps directly to your phone (just as other handheld navigation systems). Although Navigon released their iPhone App before TomTom and Garmin did, TomTom has already announced that they will release their application with an iPhone car kit enabling you to charge the phone and use it as a hands free device.