There is a
Planescape short adventure developed for the RPGA called "The Manxome Foe" in the product
TSR Jam 1999.
There are also five
Planescape novels:
Pages of Pain by Troy Denning.
Torment by Ray and Valerie Vallese. The Blood Wars trilogy (
Blood Hostages,
Abyssal Warriors,
Planar Powers) by J. Robert King.
There's also the out-of-print
Blood Wars collectible card game.
It isn't official, but there is also a fan-made
Planescape web-novel called
Fire and Dust by
James Gardner.
To clarify some of what has been said before, there are a few adventures and supplements with a planar focus, but are not
Planescape per se:
A Paladin in Hell - a generic AD&D adventure that has you going through a particularly odd layer of the Abyss to fight a deposed arch-devil in Hell.
For Duty & Deity - this
Forgotten Realms adventure is a crossover with
Planescape's
The Infinite Staircase, and the majority of it takes place in the Abyss.
Vortex of Madness and Other Planar Perils - is a generic AD&D collection of adventures taking place on various planes (a loose hook is given to tie them together).
Guide to Hell - this genericAD&D supplement details everything you wanted to know about Hell but were too afraid to ask...it even has the half-devil, something unseen in 2E (and maybe 1E as well) until then.
Warriors of Heaven - a generic AD&D supplement on how to use celestials as player-characters, and how to run a celestial campaign.