http://h34nug3iwqjf2376fmmzdhgqivckzemcujmarlptfqeqtq7vya47yyad.onion/2007/02/21/gtetrinet-multiplayer-tetris-with-some-twists/index.html
And I have checked this for years now. ;-) Servers I already mentioned the public available tetrinet servers but there are more. You can also run your own server, using for example the package tetrinetx, which provides a nice tetrinet server implementation. There is also a package tetrinet-server, but it’s more primitive.