Pentago

Pentago is a two-player abstract strategy game invented by Tomas Flodén.

Taking turns, the two players place a marble of their color (either black or white) onto an unoccupied space on the board, and then rotate one of the sub-boards by 90 degrees either clockwise or anti-clockwise.

This is optional at the beginning of the game, up until every sub-board no longer has rotational symmetry, at which point it becomes mandatory (this is because until then, a player could rotate an empty sub-board or one with just a marble in the middle, either of which has no real effect).

A player wins by getting five of their marbles in a vertical, horizontal, or diagonal row (either before or after the sub-board rotation in their move).

[1] So far Pentago has won 8 major awards, lastly Game of the Year 2007, by Creative Child Magazine.