For some years I have been aware that a detailed modern map of ancient Rome existed, with the modern street layout superimposed on it. Bits of it cropped up in this publication or that, but never referenced. Quite by accident this evening I found out what it was — Rudolpho Lanciani’s Forma Urbis Romae, a collection of plates published between 1893-1901.
Parts of it are online here, although I must confess that I’d really like to see PDF’s of the whole plates. If you burrow into that site, you do get to some decent JPG’s.