Google Earth... Hold the world in the palm of your hand. With Google Earth for iPhone and iPod touch, you can fly to far corners of the planet with just the swipe of a finger. Explore the same global satellite and aerial imagery available in the desktop version of Google Earth, including high-resolution imagery for over half of the world's population and a third of the world's land mass. With Google Earth for iPhone, you can:
Tilt your iPhone to adjust your view to see mountainous terrain
Show the Panoramio layer and browse the millions of geo-located photos from around the world
View geo-located Wikipedia articles
Use the Location feature to fly to your current location
Search for cities, places, and business around the globe with Google Local Search
What's new in Google Earth for iPhone
Performance improvements and bug fixes.
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Not really useful for me as it crashes soon after launch.
I'd wait for a more stable version which might be useful (e.g. Wikipedia access is possible).
So far Google Maps included in iPhone is my preference.