Acid Tetris is one of many many versions of Tetris available for the Windows OS, and in my experience, one of the best.
This game is not to be confused with Tetris on Drugs, and in fact, has no actual game play differences from the original Tetris. The skill of this game is all in the graphics, sound and smooth execution of the basic Tetris concept.
The music is somewhat cheesy new age synth music that nonetheless calms the nerves while not interfering with the game play. The graphics are pretty standard, although when a row or rows completes, the disappearing blocks shimmer, flip or explode. This effect is done so subtly that a player that is focusing on playing the game will hardly notice.
Also interesting is the Smiley Face that shows up to express boredom, concern or approval with the players progress in the game. Many people find this rather annoying at first, although in time they become accustomed to it.
Acid Tetris is freeware and is about a meg to download, and will work under and Windows system less then 8 years old.