Forge MTG is described as 'Forge is an implementation of the Magic: the Gathering card game with rules enforcement. Up to 8 players are supported, with control of each assigned to human or AI control'. There are four alternatives to Forge MTG for a variety of platforms, including Linux, Mac, Windows, Self-Hosted solutions and Android. The best alternative is Magarena, which is both free and Open Source. Other great apps like Forge MTG are Wagic (Free, Open Source), Cockatrice (Free, Open Source) and XMage (Free, Open Source).
Magarena is a single-player fantasy card game played against a computer opponent. The rules for Magarena are based on the first modern collectible card game - Magic: The Gathering. Represent entire series of Magarena games.
XMage allows you to play Magic against one or more online players or computer opponents. It includes full rules enforcement for over 19.200 unique cards (over 37.400 counting all cards from different editions, you can find event custom sets like Star Wars).