HearPlanet is a little audio guide which you will learn more about the landmarks you visit. It's available currently only in English, some reference monuments and cities, most of which are in the united states.
Another good point, the application uses GPS to locate you and so display guides available near you. The texts from Wikipedia and are read by a synthetic voice. Its innovation is I like the idea of reading the wikipedia article is clever.
For now this application HearPlanet is free in the Apple App Store, but will soon pay.