So the other day I stumbled across a paper in arXive called "Magic: The Gathering is Turing Complete." I don't know enough about either MtG or computer science to get more than 11% of the gist of the piece, but some of you might be interested in the cards needed to turn a game of Magic into a Turing Machine:
Of course, if you can use MtG as a Turing Machine, that leads me to ask the obvious question: Can you run Doom on it?