I personally use cockatrice, there's almost always a game available

MTGO and MTGA are the official clients (MTGA only has brawl. MTGO has commander but is also not particularly budget friendly). There are also unofficial ones like cockatrice, untap.in, xmage, forge (with some of these, you might be able to play commander, with others not). Many also play commander via webcam using https://spelltable.wizards.com/

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I've been playing some Magic recently on Forge and considering rejoining my local game store (LGS). However, it seems like the company is pushing Commander a lot, and the most recommended way to play it is through Cockatrice. Nevermind, I'll keep playing on Forge, and maybe I'll try Cockatrice. I'm just not excited about playing a format where "Bobsponge" is a thing. It makes me wonder, what are they even doing with this game?