I created an old sewer system for my SCAP game, mainly because I wanted the PCs to have alternative entrances to house Rhiavadi and the temple of Wee Jas.
A (simple) map can be downloaded here:
http://therpgenius.com/shackledcity/AreaMaps/General/tabid/97/Default.aspx
I'm using the "Sewer stronghold of the thieves guild" Map of Mystery from dungeon magazine 128 as the Last Laugh's guild house, connecting to the Slippery Eel tavern above via the tavern's cellar.
Has anyone of you done something similar?
I used several underground complexes, like Jazidrune, Drakthar's way, and a few of my own creation. I avoided sewers though. I went with the whole waste washed straight into the lake thing. So no sewers in my city!