It feels like one of those obvious ideas that's not quite practical: Tetris with letters, with columns disappearing as they form words. Certainly, a lot of tweaking has gone into making it playable: each letter's probability of appearing is weighted (as in Scrabble), a Q is usually followed by a U, there are occasional 'eraser' blocks to deal with truly unplayable letters, and the whole game is played in a water-filled well (allowing even more combinations to be formed and disappear as the old blocks sink under new ones.) Thankfully, all this work pays off: Wordtris' carefully designed balance makes it at least as addicting as anything else in the Tetris series, while being much more intellectually stimulating.
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See also: Tetris Gold
Color version requires 256 colors.