This is simply to announce that the full Sigil NPC List is now available for download in this site's dowloads page.
The document is currently only available in Excel format, though Clueless is kindly working on setting up a online search format similar to the List of Gods page. Many thanks to her! This page will soon be followed by another download page for the Sigil Locations list and hopefully a page to download the Sigil map I put together.
I'm aware of what the Sketchbook and Conspectus are even if I've never seen either. I don't imagine that many of the remaining sources will yield many new Sigil NPCs and Locations since they're dedicated to other planes, but you never know when a throw-away quote will pop up in a Planescape product so it's best not to rule anything out without checking it first.
Besides, I'd love an opportunity to flip through the sketchbook. Does anyone have a copy?
I actually have access to most of the unindexed material, the only exceptions being the Player's Primer to the Outlands boxed set, Monstrous Compendium Appendix III, Planescape Conspectus, The Planescape Sketchbook and the Torment novel. It's simply that, after working my way through the first 20+ sources, I started to lose a little steam. I figured 700+ NPCs and 250+ locations culled from all the primary Planescape sources was adequate for an initial online posting. I do intend to eventually work my way through the remaining sources (though I'm dreading tackling the Blood Wars Trilogy; reading through it once was challenging enough).
I don't know enough about Excel or Microsoft works to help you. If someone can suggest a better file format for the document then perhaps I can resave it and post it online...
Very nice work, Ambrus. I had something similar but yours puts mine to shame.
Just one recommendation, if you don't mind: could you put the ward name in a separate column from Commonly Found? That way a GM could quickly sort the NPCs by the ward his PCs are in at the moment.
A text file would be nice. I tried just renaming it as a text file, but I end up with a lot of formatting gibberish, and the columns are out of order.
You could try saving it from Excel using the Text File (tab delimited) option, though the great width of the original makes any text format unwieldy. If I have time I'll try to convert it into a searchable PDF.
And of course as Ambrus mentioned I'm working on turning this into a database like the God's List to help make searching and indexing material much smoother as well. 
Since Clueless has graciously setup a page for the Sigil Map I thought I'd ask for opinions in regards to the file size and format(s) which would be of use to the most people.
The map was originally created in Illustrator (all the text is vector based) with an imported hi-res RGB Photoshop file for the background image. As is the file is 32" wide by 22" high at 300 dpi. When it came time to print it, I supplied my girlfriend with a hi-res PDF which weighed in at about 30 megabites. She printed it up on a large format inkjet printer she has access to and now I have a beautiful big full-color map of this great fantasy metropolis. 
I know not everyone has access to a large format printer or high-speed internet so I figured it'd be best to ask to find out what people want to see and then tailor the download file to match. So what size/resolution/format would be best? What's better; Jpeg or PDF? Are multiple download options a better idea? If so how many?
The Planescape Conspectus is basically just an ad that came with Dragon Magazine, and has no substantial new information. It's a bunch of images from Planescape products crudely Photoshopped together on a fold-out poster, with some explanations of what Sigil, the factions, and the Outer Planes are.
The Planescape Sketchbook was a book of conceptual art Dana Knutson made for the campaign setting available at Gen Con one year. There might conceivably be some drawings of Sigil NPCs in it that might be indexed, but that wouldn't be useful to many people.
The other sources you mention seem worth adding to the index. Off hand, I know there's at least one major Sigil NPC in the PSMCIII, and it'd be fun to add characters from the Torment novel (some of whom also appeared in Dragon #264). I don't have the Blood War trilogy or In the Abyss, but I can do the others.