The latest version of Google Earth includes an option to explore 3D-modeled ancient Rome. Be warned that this thing is quite a resource hog. To activate the modeled city, upgrade GE, select "Ancient Rome 3D" in the Gallery layer (you might have to set the language preference to English for this option to show up) and zoom close in on Rome.

You should see lots of yellow markers of the Rome Reborn project. Open one of them and there should be links to the ancient terrain, landmarks & buildings. Downloading the data may take a while.