Natural ones: Giant, Tunnel, Racing, Hunting, Sword Spiders, and phase Spiders

Unnatural: Wraith (These are undead), and any spider found in company of drow, behemoth.

Saved you time Oro :P

Where did that information come from though?

If its out of your butt, you can keep it there!

If it came from a DM thanks a million.

plus skullspider...

I know for certain that TNVW's information is inaccurate.

Saved you time, too!

Druids can PolyMorph into Sword and Phase Spiders, logically these are natural.

Giant spiders are pretty much found in every NwN game, in forests etc...

wraith spiders say they are undead. (descriptor)

Phase spiders are actually magical beasts.

Tunnel, Animal companion.

Racing spider, natural, found in most places in underdark and druids don't bother them normally.

First off, phase spiders are magical beasts. Yes, wraith spiders are unnatural as they are undead. The rest of the spiders I believe are -supposed- to be natural but have been twisted by the Drow and now they act/resemble unnatural (I am sure through ig events you could try to change this).