Augmented reality might be the future, but my favorite application of it yet transports you far into past. StreetMuseum—an iPhone app from the Museum of London—overlays four hundred years of historic images on today's city streets.
StreetMuseum makes creative use of Google Maps and geo-tagging to show users how London used to look. You can use it to check out pictures and info about nearby historic locations, which is has more of a straightforward walking tour feel. But the fun starts when you're actually standing in front of a location in the database. That's when the AR "3D view" kicks in, with views that may look something like this: