It is a three.js extension to display a fully procedural city based on "city" , a demo recently released by @mrdoob. You can try a demo live and check its source. Here is a more basic example and its source.

NOTE: see full implementation details in "How to Do a Procedural City in 100 Lines" post from learningthreejs blog.

You can install it manually or with bower. for the manual version, first include threex.proceduralcity.js with the usual

<script src='threex.proceduralcity.js'></script>

or with bower you type the following to install the package.

bower install -s threex.proceduralcity=https://github.com/jeromeetienne/threex.proceduralcity/archive/master.zip

then you add that in your html

<script src="bower_components/threex.rendererstats/threex.rendererstats.js"></script>

Super simple, just create an instance and it will return a THREE.Mesh

var city  = new THREEx.ProceduralCity()
scene.add(city) 

The demo live contains this city plus a ground, a first personn controls and a fog. This is rather cool result for as little effort.