Tetris Online (Japan)

Tetris Online is an official online Tetris game for Microsoft Windows 2000/XP.
 * No ARE, IRS, IHS is present.
 * T-Spin recognition is nearly 3-corner T, but T-Spin with Wall kick displays as "T-Spin Mini". In single player mode, there is no difference to Tetris DS. But in VS modes, "T-Spin Mini" sends fewer garbages than T-Spin (no kick).
 * DAS is very slow and useless.
 * Need rapid left key or rapid right key to shift quickly.
 * Key assignments cannot be changed.
 * Only 2 next pieces are visible.
 * Randomizer is different from other games. It uses variant of Random Generator. It is regular 7 piece bag + 1 random piece system. Source
 * Time Attack, Challenge, Marathon, VS COM, Multi mode is present.
 * Time Attack: Well known 40-lines time attack mode.
 * Challenge: 10-minutes score attack mode. Unlike to the Tetris Zone, game ends immediately if the level timer runs out.
 * Marathon: Standard Tetris mode. Game ends in level 15. Required points to increase a level is much shorter than other guideline compatible games.
 * VS COM: Battle against AI-controlled player. The AI player uses regular 7-piece bag system and 6 previews, so you must fight in unfair situation.
 * Multi: Battle against other players.
 * (Practice): It is not a real game mode, but you can play it in Multi mode. (After you die, or before the game starts) Game continues until the current match ends or the match starts. The gravity becomes 20G in level 18.

Official Site