The Museum of London has just launched an iPhone app that makes use of its extensive art and photographic collections as well as geo tagging and Google Maps to guide users around London where, via the iPhone screen, you can look into the past! By overlaying old photos, The Museum of London gives you a unique experience, making you want to explore the hidden history of London that surrounds you.
This idea may have been inspired by the Flickr group, Looking into the Past, which has an amazing collection of photos within photos.
Museum of London: Streetmuseum is available as a free download from iTunes now. You'll need to own a 3GS to take advantage of the augmented reality option.