Now someone needs to use this to make a passable Magic AI. Shandalar is ancient at this point, but it's still the best attempt I've seen, and the only Magic computer game I still play.
http://en.wikipedia.org/wiki/Magic:_The_Gathering_–_Duels_of...
It focuses on premade decks with limited means of customization, rather than open freeform deckbuilding. (Of course they want you to start buying Magic proper to do that.) But it is the only official way to play modern Magic against an adequate AI.