Hawk Wargames, makers of the hot new 10mm sci-fi game Dropzone Commander, is coming out with a lovely line of buildings made from cast resin tiles. The tiles are made in either commercial or residential sizes, and have a sort of vague 1930's art deco sci-fi look to them that I really fancy. By mixing and matching the various lobby, wall, and cornice tiles, you get a pretty unique look for each building that still looks coherent. Tiles are 36mm wide by 17mm or 21mm depending if they are commercial or residential, and the ground floors are double height.
Here are a few pictures linked from their site.
Expensive at roughly 75p a tile, but they look great, and while filling up a town would be costly, they could probably be used to "sci-fi" a N scale cityscape pretty easily, and certainly a backdrop of just building facades could be done reasonably.
Wish someone made this sort of terrain in 6mm... although I am afraid I would have to buy quite a lot of it if they did!