Well-designed transit maps can tell us a lot of useful things, but there's always a physical/psychological limitation to how much information they can carry. Past a certain threshold, things become too dense for more people to figure out, or you simply run out of space... But with augmented reality, the same map can dynamically change and show you different things, depending on what you're looking for.
This is what the iOS app called Tunnel Vision does for the New York City subway system. Point the app at one of the ubiquitous transit maps, and it overlays information like turnstile and schedule data from the MTA or US census data.
Oh, did I mention that the app is also free? You can get it here.
Other cool stuff with transit maps: This, below, might look like just a regular circuit board, but it's actually a map of the London Underground! Click on it to learn more about it.