There's a pretty cool script for Blender.  It allows you to create a city with a click of a button.  Hopefully it will allow you to have a lot of customization as well.  Usually with these single click programs, the end result will always look the same.  On the other hand if you need to fill up your scene (especially the background), this is a great tool. the script is called the Suicidator City Engine 0.4.  That's a lot to say.  When you look at the video, it seem to be a great arsenal for you Blender toolset.

More information and download: http://arnaud.ile.nc/sce/