Google have just announced a new resource that is available through Google Earth - a virtual ancient Rome!
Google have worked in conjunction with Virginia University, who's researchers have also developed the Waters of the City of Rome interactive website.
A BBC news item on the project is available here.
The Google Earth ancient Rome page is here.
(Update... it does work! You have to click on one of the ancient Rome icons and then download the different layers, starting with 'terrain'. The terrain layer loads quickly, but the others take ages!)
Viginia University's Rome Reborn site is here. (with less cartoony imagery than Google!)