The Museum of London has got a free app for Iphones, etc. You can read about it here.
It has a map of London where you can click on different locations to see photos of how they looked in the past and read about them, and if you have an Iphone you can hold the camera up to an actual street in London and it gives you historical information about it. I’ve only got an Ipod Touch so I haven’t tested this part of it but the map bit is great.
You can download it from Itunes by searching for Streetmuseum, or directly via the App Store on your device.
Wonder if other regional archive offices would consider making something similar?