Museum of London - Street Museum
The Museum of London have created a great iPhone application to showcase hundreds of images from its collections.
You can use the included Google Map or use the iPhone's GPS to pinpoint your current location. You can then view images from the museum's collection that are in your vicinity. If you hold your iPhone up you can then look through the screen and see the historical image superimposed on the present day view.
The Creative Review have been testing the applications' augmented reality function. They have some truly astonishing screenshots of historical photographs overlaid on the modern view as seen through an iPhone.
This is probably the closest you will get to real time travelling until Dr Who drops in.