Though it's really not much of one yet. Thus far, I think it only has portals to the city of Mitrik in Veluna, the World Serpent Inn, and the demiplane of Tol-Kan-Zeeb (described in the November 1988 Dragon Magazine, this isn't the proper name of the demiplane, but of the djinni mage who created it). But I think Elayne Mystica, Bigby, and the United Artificers Guild are busy crafting more.
The city of Raven's Bluff in the Forgotten Realms might be more of a planar city, though most of its portals are hidden in the catacombs beneath the city, forgotten by those who dwell on the surface.
Inphirblau was mentioned, though it only got a line or so. And Union was alluded to, in the demiplane of Haven. Unless that's supposed to be a completely different mercane demiplane-city.