Having an iPhone with GPS and location-aware applications is incredibly useful when exploring a new city. For example:

All these apps have links to Google Maps, and when that opens you can instantly plot a route from your current location.

The main thing missing is a 'plot all these on a map' application, or even directions, so you can see which things are close together/in a certain area.

Twinkle has tweets from people nearby, but there aren't many people using it at the moment; when they are it will definitely need some filtering.