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Streetmuseum: Londinium iPhone App
Streetmuseum: Londinium is a joint effort by the History Channel and the Museum of London. As its name suggests, the app is basically an interactive history lesson about Roman London. Via its mapping interface, it lets you investigate old Roman sites that have been found around the UK's capital.
All the sites are overlaid on a modern map of London, so you can see how the city has developed over the years. You can even perform your own virtual archaeological digs using your fingers to dig in the earth, while blowing on your phone's microphone will remote dirt. There are augmented reality features too, and walking guides that use your handset's GPS abilities to keep you on track.
It's a bit gimmicky in places, but Londinium offers up plenty of information and is largely a fun way to discover the Roman origins of many of the capital's most famous streets and landmarks.