Freen in what sense? Free as "you don't have to pay to play in them"? I think all Minecraft servers are free in that aspect. Some may ask for donations to keep the server running, but the vast majority of them are completely free. Or do you mean "free to host"? If that's the case, I guess that you can always host one yourself in a spare computer at home, but if you want a good service, you'll have to pay for it.
If you mean if Minecraft itself is free, no. You have to buy the game to play online.
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