I suggest not using the black widow spider skin previously posted alongside this spawnclass mod, as you might get confused between the spider types (though both mods are compatible).
unzip and place full file structure into your %appdata%\Runic Games\Torchlight\mods
download: http://www.torchlit.net/latest-mods/item/download/111.html or http://www.runicgamesfansite.com/torchlightmods/file/341/download/342
This mod alters a few spawnclasses so that the black widow gets added into the random monster lineup in the mines and such (let me know if I missed any common spider spawnclasses). This mod should also be compatible with TorchEd for the other modders out there who want to add the black widows to their dungeons. enjoy.
they do more damage than your average spider and are a bit faster, so be careful...

If all you want is the simple reskin of the spider then that is still available here:
http://www.torchlit.net/latest-mods/item/download/110.html or http://www.runicgamesfansite.com/torchlightmods/file/340/download/341
original skin - note the new monster is smaller than in this image:



