Blizzard has a new feature on WoW.com that allows you to track six different statistics. You can see the most auctioned items, most created items, most gathered items, most dangerous NPCs, most completed quest and most looted items. While this is not exactly the most useful information, it sure is cool to browse around.
And the most gathered item yesterday? That would be copper ore...and I imagine it will keep the top spot for a long time to come.