The Museum of London's Street Museum app has always been among our favourites. It uses augmented reality to overlay old photos of London over your phone's camera view. It's just been re-released with 103 new locations and improved functionality, and lets you order prints of favourite images.
To give you a taster, the museum has worked on these stunning images of old and modern London combined. This mixing of eras is something of a trend at the moment, but the Museum's efforts are the best we've seen.
Street Museum 2 is available for Android and iPhone, and is totally free.