Lot of hate talk about mill counters, or at least one guy who I'm not sure has ever played against a mill deck. I play regularly with a group of friends and one of them loves, loves, loves mill. All of the decks one a 1-of copy of some card that shuffles if it's put into graveyard (Eldrazi Titans,
Blightsteel Colossus, etc. I had a bunch extra lying around so i just threw them in). His general answer to that is
Leyline of the Void or
Rest in Peace.
Which is why it shocks me that nobody ever talks about
Wheel of Sun and Moon, though. It's my favorite mill counter. yeah it's an enchantment so easy to kill, but if it's out there it kinda derails the mill until it's dealt with. One of my favorite cards. Also, the art is amazing.