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.

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.

It's almost ridiculously good. Donate if you can!