While Sigil is the center of the Planescape multiverse, it is far from the only fictional city at the dimensional crossroads. What other such cities exist? Here are a few:
Tanelorn: The eternal city from Michael Moorcock's Eternal Champion stories (Elric, Corum, etc.). It exists somewhere in every dimension, though it appears differently.
Amber: The home of the characters in Zelazny's Amber series. Other dimensions are shadows of Amber.
Cynosure: The cross-dimensional city of the GrimJack comic series. You can change dimensions by crossing the street. This one reminds me the most of Sigil, with its gritty feel,questionable morality and fatalism. Not the same as the Cynosure where the FR gods meet.
What other such cities are there? What are their characteristics? Material could be from fantasy fiction, other game systems, comics, etc.
Metropolis in the Kult Rpg. Every city is a shadow of metropolis and sometimes the barriers between our world and Metropolis is torn and from these oppenings Metropolis poures in.