Sergio's Vestiges (http://nifflas.lpchip.nl/index.php?topic=5192.0) tilesets are great, but you know of these already. Here's some public-use cave stuff I found...
Edit: There were some nice tilesets in the level Darkness by Pick Yer Poison. The links are dead but you can see the tiles to the right in this screen shot (http://nifflas.lpchip.nl/index.php?topic=1377.msg14766#msg14766) I found.
Thanks guys; those are pretty awesome; there are some I definitely want to try out there.
It's great to be part of such a supportive community :)
EDIT: Screenshot!
The whole level revolves around that cube at the top.