Magic: The Gathering Online
There are no monthly fees for playing as one only pays for new packs of cards and can play with or trade them at one's discretion.
Many popular modes of play are supported such as constructed, limited or booster draft which one can play on casual basis (head to head or multiyplayer) or at official tournaments.
The single most important feature is that every rule is properly handled by the program, so one can be 100 percent sure of the way a duel is resolved. This makes the game enjoyable for unexperienced players as well as veterans.
An interactive tutorial is available along with a trial mode free of charge.
Original Magic Game Design
Game Design and Development
Wizards R&D Representaion