According to this article in today's New York Times, Google Earth has gone Roman! Ancient Roman, to be exact. Now you can virtually explore Rome as it was under Constantine the Great!
Rome is full of ruins, but it's also full of modern structures (and by "modern" I mean from the Renaissance forward). When you're there, even if you've studied the ancient city, it's extremely hard to judge geographically where you are. The above image shows the Circus Maximus (the long white structure), which is awesome because when you see it now it's essentially a long skinny field. Very exciting.