TetriNET is described as 'Multiplayer online Tetris game for up to six people, developed by St0rmCat in 1997 that supports team play. The last official version is 1.13. The game was originally developed for Windows and was later ported to other systems' and is an app. There are more than 25 alternatives to TetriNET for a variety of platforms, including Android, Windows, Linux, Mac and iPhone apps. The best TetriNET alternative is KBlocks, which is both free and Open Source. Other great apps like TetriNET are Tetris, Myrntris, Block Drop and TETR.IO.
Blocktrix supports the TetriNET protocol with more modes, modern interface, and more.