image above: a basic city generator coded for presentation & workshop at the EUSS2012, reflecting existing building types and policies in the form of a series of image maps that can be changed to see possible spatial results from the interrelated layers.

A list of existing city generator:
Comprehensive
City Engine – procedural road generation, plot divisions, buildings
Subversion city generator – procedural road generation, plot divisions, buildings

Partial
GhostTown (3dmax) – procedural plot divisions, buildings with texture
RenderReady (Maya) – procedural buildings with texture
Kludge City (Maya) – procedural buildings with texture
Suicidator city generator (Blender)
and more (such as sity and other indivdual attempts)

So there are already quite a lot of existing generators out there. The problem is that most of these are designed as final products – producing visualisation or environment for game / movies. While we can use python and rules file to control certain aspect in City engine, there are no ways to access the core functions such as the algorithm for lot divisions or path generations (correct me if I am wrong).