Good topic to start, Terra. Depending on interest, I'll add it to the Thread of Threads.
As for cities and towns on the Planes:
-There's obviously Sigil, which has its own thread already.
-Gate-towns. Depending on the direction we choose, they can either be the "Suburbs of Sigil" or stand-alone cities in their own right. Heck, they can even be both!
-There are three major burgs on the Inner Planes: the City of Brass, the City of Glass, and the Great Dismal Delve. There was also going to be a City of Clouds, but Lucasfilms sued
-On Baator, the layer of Dis is just one big hell-tropolis.
13/6
Edit: Changed the title from Planar cities to Towns and Cities of Urban Planescape.