I'm new here and I have absolutely no idea about any of the 3E stuff that was written for Planescape. I was a great lover of the game but I assumed that the setting had died with Faction War and that any attempt to continue it after that story was going to be an exercise in futility.
So, I'm really a Clueless Prime about what exactly is here and what I should look into.
Also, what the community is all about. But I'm going to try and recruit my friends into it.
If you mean official 1st-party stuff, there are only a few things, mostly a smattering of articles from Dungeon and Dragon magazines. I cannot remember all of them, but I do have a couple of the issues.
Dragon 287 did an update on some of the Factions, and so did Dragon 315. Dragon 354 did a new version of the Great Modron March.
Sigil is still mentioned in the Manual of the Planes and Planar Handbook.
If you mean 3rd-party stuff, well, the chapters that have been redone right here on Planewalker are quite good.
For the future, and 4th Edition D&D, the future is uncertain. The cosmology has been redone and the Fiends have also been given a facelift. What we can (or cannot) do to adapt to 4th edition is already being talked about on these forums.
I'm smart enough to know that the only Planescape stuff that's been officially done is the Expedition to the Demonweb pits and am primarily interested in the semi-official fan stuff.
I was avid reader of the Kargatane material after the fall of Ravenloft.
So, what's good and what's not? What should I be looking for.
Also, can I join up in helping?
Oh where to begin 
If you haven't checked it out yet, the following page contains the main PS3E document that Planewalker's been working on since the beginning.
/downloads/products/released.php
Other than that, we have several projects going on right now in various stages of completion, and any help is greatly appreciated.
Browse around a bit, catch up on the recent planar events we've managed to catalogue, and enjoy yourself.
Welcome back to the planes, cutter!
The 3E Campaign Setting really is awesome, but for general information on specific parts of the planes, you really can't beat the Encyclopedia.
Cool, I'll definitely start reading the Planescape book that you've been working on all this time. Are there plans for any others after this or other supplements? I'm curious if there's any sub-books of note too.
I'm sure they're all good but I was curious for a place to start.
Thanks!
There's lots of various projects we're working on. We'd like to be able to pick up where 2nd ed left off an finish writing Parts 2 and 3 to Faction War as well as many other planar adventures and developments.
Three projects just off the top of my head (plenty of room for more if you've got ideas and the will to keep the posting alive):
Shattered Dreams - PSCS Module
Of Darkest Grey - Hordling and Night Hag Project
Ortho - PSCS for The Harmonium's Homeworld
How is it not one? Unless you're *that* fond of the rather clunky wiki formatting language?
Actually, I've been FORCED into printing up and reading the Planescape 3E now.
(Not that I took much forcing)
Primarily, because my 3E players have never read the original and are reading it here.
Yes! New converts. . .
muahahaha
Rad any original stuff you get your hands on too. Ignore the 2E rules and stats but use the flavor. May I suggest the Planewalker's Handbook for this purpose. All of the 2E stuff could be downloaded cheap ($5) in PDF format at drivethrurpg.com.
NOTE: By suggesting that one looks at the original material, I am in no way suggesting that the PSCS 3.5E is anything but excellent. Keep up the good work!
The PSCS has barely scratched the surface - I'll freely admit. But it *is* intended for new bloods - so let me know if it looks like I've either missed something or you and your players have any questions that desperately need answering.
Heh. 3e stuff. Hehe. In all seriousness, a lot of it contradicts 2e cannon or is just too vauge. As far is I've seen using the 2e flavor from the books and the 3.5 rules and monsters (wherever you can find them, fiends, etc. are all mixed up in different books) makes good sense unless you want to try and reconcile some of WOTC's 3e changes with the 2e stuff.