Recommending Google Earth is a pretty obvious thing to do - in fact, I surprised myself when I realised I was yet to feature it on this site. There probably isn't much need for explanation, other than to say that Google Earth for Android takes the desktop experience of Google Earth and seamlessly ports that to your tablet or mobile.
In fact, with great multi-touch gesture support, using Google Earth actually feels a more natural for android than PCs. One of the great new features is 3D rendering of popular cities and buildings. With the ability to "fly" through these locations, using Google Earth is the next best thing to actually visiting a place.
In short, Google Earth is an essential app for any android users.
In the words of the developer:
Explore the world from the palm of your hand with Google Earth.