The Medieval City Plan Generator. It's the free online tool you've always wanted. It doesn't create maps of actual medieval cities--only nice looking maps of imaginary cities, with the ability to add plazas, castles, rivers, city walls, and even shanty towns. Equipped with all kinds of options for lettering and colours.
What can you do with this?
This application generates a random medieval city layout of a requested size. The generation method is indeed rather random, the aim is mainly to produce a nice-looking map, not an accurate model of a city. Perhaps the developer (watabou) will use the code again in the future as the basis for a game.
Also read: Generating a new world map