Neverwinter Nights 2: Build Your Own Sewers!

Yes, you read it right! Though confirmed that a 'sewer tileset' will not be included, the question was asked if, using other tilesets, one could still create a sewer. Brian Lawson gave this answer:

You can definitely make your own sewers in this game. Adam and I every once and a while take one of the more "sewer like" interiors and slap down some shallow water just for the heck of it and it looks simply unbelievable. Reflections of the pillars, refraction of the drainage grate and floor beneath the water, etc...

I think you'll be pretty happy with it.

There you have it...a recipe for sewer building.