Of particular interest lately is an 8 mana white enchantment (6WW) called Divine Intervention. It saw a decent increase recently before coming back down a bit before yet another surge. It hasn't spiked but is sitting between 10 and 15 dollars. What's particularly interesting about this card is its rather unique effect.
There are several cards in Magic that serve as alternate win conditions. But Divine Intervention, after it has two intervention counters removed from it, actually ends the game in a draw. From a competitive standpoint, this doesn't seem like a very fun card. It has seen some fringe play in Phelddagrif "group hug" EDH decks as a way to simply let no one win.
While it doesn't seem like you'll be seeing Divine Intervention becoming a staple anytime soon, seeing what a mediocre enchantment like field of dreams has done, it can't hurt to find a good condition copy or two of this. All you need is for the supply to dry up all of a sudden which with these very old cards can happen quite easily.
If anything this is just a cool gem from early Magic. It's a cool one to have and in EDH a great way to end a Commander game no one really wants to play anymore.
(Update 7-8-2017: Divine Intervention did later see a spike to $40 in March of 2016 before settling around $30-35. Being on the Reserved List, it's still a good investment.)