Tetris

Rotate and drop falling blocks to clear lines. The classic falling-block puzzle, speeding up as you level.

Alexey Pajitnov wrote Tetris in 1984 at the Soviet Academy of Sciences, and the tangled rights that followed — between the Academy, a Hungarian licensee, Mirrorsoft and Nintendo — became one of the most famous ownership disputes in the industry. Pajitnov saw no money from it for over a decade.

Modern play is defined by the seven-bag randomiser: pieces are dealt from a shuffled bag of all seven shapes, so you can never get more than twelve pieces between two I-pieces. That guarantee is what makes deliberate four-line clears a strategy rather than a gamble — you can build a nine-wide stack and know the long piece is coming.