Crothian said:
I don't own Underdark so I didn't want to list it not knowing exactly what is in it.
Underdark Adventures has some cool ideas, but it might just be Mike Mearls weakest book I've seen. To me it was very hit and miss, unlike most of his books.
I'll ditto this. Underdark Adventures is the closest in nature to the old Dungeoneer's Survival Guide (which is the only 1e book sitting on the shelf within reach, with my 3.5e books, so that should tell you what I think of it), but is (sorry Mearls) pretty dull. I haven't looked at it recently, but I recall feeling like it was nothing new - duergar are evil, drow are nasty, yawn yawn. Not alot of space given to anything really new and exciting.
Underdark, from WotC, has some great ideas and great material. Your utility will depend on your adherence to OGC, obviously.
I haven't looked at Dungeon or Dungeoncraft recently.
Goodman Games put out "Monsters of the Endless Dark", a bestiary of underdark monsters. I'll guarantee they're not what your players are expecting.
Expeditious Retreat Press has "Underground Monsters" (or somesuch) as a pdf, and possibly a print product. No pretty pictures, just lots of underground OGC monsters.
I do find it interesting the most unique environment in d20 is so neglected in products, though.
Cheers
Nell.