Sigil

The city of doors, reachable only through planar magics. No powers are allowed in the city and an entity known only as the Lady of Pain controls access to it making sure that no dieties come through. Anything can be found here. Anything. It's unique geography means that if you look up from one area of city you can barely make out the streets in the distance on the other side. People from all sorts of planes and planets can be found prying their trade here. If you know the way in, it's a definite must-see. Just don't get trapped in the mazes.