You could try looking for RPG Citymap Generator. It isn't the greatest software, but it is free. Also, have seen a couple of links here on the forum for web-based city generators. Probably in the software or resources sub-forums. Those, along with nearly any graphics editor will get you headed in the right direction. As for being able to zoom in and what not, depending on how detailed you want things to be, you might also look at some AutoCAD based software.