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If you want to play Magic against an AI on a computer I can recommend Forge: A very solid piece of work in it's class. Has quest/camapign modes, shops (free), pretty good AI and a million of decks and cards you can play with or against. Needless to say, it's not multiplayer...

When playing around with it a lot of ideas come to mind about what makes a good and bad game/cards... could be worth trying out just for that