Tetromino ice cube tray
$10 from Thinkgeek.
It's tempting to buy a few, then set up an aluminum sheet at an angle and actually try playing Tetris by sliding them down. As a completed line will not vanish automatically, a sense of accomplishment could be had by making the tetrominoes out of certain beverages and eating them instead.
Ice Blox [Think Geek]

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Such wasted space in the tray! Haven't they ever played Tetris?
Tetris jello shots!
Why is the picture a 3D render?
I can hardly wait for the Pentomino version!
"a completed line will not vanish automatically"
They will, but rather slowly, so long as the temperature's above freezing.
So will uncompleted lines, but that's a mere technicality.