Shemeska said:
Faction War was not the last published PS product, but it was the last to take place largely in Sigil (the Inner Planes box was the last published IIRC).
That's right, although Inner Planes is a book, not a box (sadly, IMHO!). Here's a list of Planescape products by date:
Planescape Campaign Setting (Apr 1994)
Eternal Boundary (May 1994)
Planescape Monstrous Compendium Appendix (Jun 1994)
Planes of Chaos (Jul 1994)
Well of Worlds (Sep 1994)
In the Abyss (Oct 1994)
Deva Spark (Nov 1994)
Planes of Law (Jan 1995)
Fires of Dis (Feb 1995)
Player's Primer to the Outlands (Apr 1995)
In the Cage: A Guide to Sigil (May 1995)
Factol's Manifesto (Jun 1995)
Harbinger House (Jul 1995)
Planescape Monstrous Compendium Appendix II (Sep 1995)
Planes of Conflict (Nov 1995)
Something Wild (Feb 1996)
Uncaged: Faces of Sigil (Mar 1996)
Hellbound: The Blood War (Jun 1996)
Planewalker's Handbook (Aug 1996)
On Hallowed Ground (Sep 1996)
Guide to the Astral Plane (Oct 1996)
Doors to the Unknown (Nov 1996)
Faces of Evil: Fiends (Aug 1997)
Great Modron March (Sep 1997)
Dead Gods (Oct 1997)
Planescape Monstrous Compendium Appendix III (Mar 1998)
Tales from the Infinite Staircase (May 1998)
Guide to the Ethereal Plane (Aug 1998)
Faction War (Oct 1998)
Inner Planes (Dec 1998)