If you use Windows and a PC, download the free Roleplaying City Map Generator, which is a great tool to spec out your city, or at least give you some ideas on city layout for varying kinds of situations. The generator offers parameters for African village, thorpe, viking village, old west town, small towns, large towns, cities and large cities. There are parameters for medieval town/cities and modern town/cities. You can make adjustments to size of streets, size of buildings, size and makeup of city walls. Once parameters are placed, hit 'go' and it randomly generates a community based on your parameters. If you don't like the result, hit 'go' again and again, until you get something you like.

I never use the results verbatim, rather only getting a good idea on how to present this or that part of the street plan, though this is a really nice tool for city plans.