Two players take turns placing X's and O'x within hexagonal cells, only one symbol per cell. Two cells are connected if they share an edge. The winner is the first player to occupy a connected group of cells, one of which is on a side of the board and another is on the opposite side of the board.
HEX was written by James L. Murphy and published as type-in source code in Creative Computing Vol. 4, No. 5, September/October 1978.