There's no app for that
Apps I wish existed for the iPhone 3G:
1. A good, free voice dialer. I used Say Who when I was walking or driving. It was accurate, quick, and easy to use. Now that it is gone, the other voice dialers don't even come close for accuracy.
2. A simple compass that works when I'm walking. There are compass apps that work while I'm driving, but I would really like something when I'm standing still or out hiking.
3. A simple search of the entire contents of my phone. The default search does not look at the notes I have inside contacts. The Search app does a great job with Contacts, but does not look at my Calendar or Mail.
What app do you wish existed?