Just want to give Lichess a shoutout here. They are fantastic, provide great content, have things for free that you need to pay for on Chess.com, and a fantastic amount of variants.
Even better, the level of play in those variants, like 960 or Crazyhouse, is MUCH higher on Lichess than on Chess.com.
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It's free, it has the same features as commercial servers, it's open source, developer-friendly, with no ads (not even on free accounts) and a transparent corporate structure under French law.
Even better, the level of play in those variants, like 960 or Crazyhouse, is MUCH higher on Lichess than on Chess.com.