The only big one no one has mentioned is Queen of Lies, from Malhavoc. The hard copy version is published, I believe, by Fiery Dragon. It's for 3E (and set miles below Ptolus, incidentally) and by Monte Cook.
If I recall correctly, there are two versions of Queen of Lies...the purist version, and another one.
I believe they are of very disparate prices on the "out of print" sellers.
Also, another recollection...Mongoose's "Quintessential Drow" isn't so hot...not horrible, I'm not sad I own it, but I wouldn't buy it for a drow campaign (unless pcs were drow).
It is no where near as detailed as the printed products people have been suggesting here, but my Ancient City of Khor is a bit of a unique underdark location. It is an independent free city in Greyhawk's underdark (although you could place it in any campaign world) that is not under the control of the drow, and that I use as a jumping-off point for a completely-underdark campaign. You'll also find on my website the Dragonsfoot community project detailing the rest of the underdark map from the old D-series of EGG modules (reprinted as the Queen of the Spiders supermodule).
As for published product, I'd personally recommend Queen of the Spiders for an old-school style location, and the 3E Drow of the Underdark for a new-school sourcebook on drow society (but not a "location").
Denis, aka "Maldin"
Maldin's Greyhawk http://melkot.com
Loads of edition-independent Greyhawk goodness... maps, magic, mysteries, mechanics, and more!