There are several online variants that you could play. Obviously there is MODO, but that costs money. Is it worth the money? Sure: It has rules enforcement, constant tournaments, and a huge player base. However only really if you don't plan on playing too much irl.
Besides that there is Cockatrice, OCTGN, Apprentice, Lacky CCG, Mindless Automation (works on mac iirc), and MWS (used at Magic-League). There are several more, but those are the ones off the top of my head.