Nullpomino

NullpoMino is a cross-platform, open-source Tetris fan game written in Java providing three versions: One using Slick, another using sdljava, the last using Swing. The game also simulates many Tetris games, including the Tetris: The Grand Master series, and a few other games such as Dr. Mario and Puyo Puyo, and also contains a utility for creating your own custom "rotation systems" and rules to create a unique or preexisting version of Tetris.

NullpoMino is one of the few Tetris games, official or fan-made, with a truly cross-platform online versus mode.